SC TO-I

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SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE TO
Tender Offer Statement under Section 14(d)(1) or 13(e)(1) of the Securities Exchange Act of 1934

CEPHALON, INC.
(Name of Subject Company (issuer))

CEPHALON, INC.
(Name of Filing Person (issuer))

2¹/2% Convertible Subordinated Notes
due December 2006
(Title of Class of Securities)

156708AD1 and 156708AE9
(CUSIP Numbers of Class of Securities)

John E. Osborn
Senior Vice President, General Counsel and Secretary
Cephalon, Inc.
41 Moores Road
Frazer, PA 19355
(610) 344-0200
(Name, Address and Telephone Number of Person Authorized to Receive
Notices and Communications on Behalf of the Person(s) Filing Statement)

Copy to:
Pran Jha
Sidley Austin Brown & Wood LLP
Bank One Plaza
10 South Dearborn Street
Chicago, IL 60603

CALCULATION OF FILING FEE

Transaction Valuation(*)	Amount of Filing Fee
$508,706,250	$59,875

(*)

Calculated solely for purpose of determining the amount of the filing fee and based upon a purchase of $521,750,000 principal amount of Cephalon's 2¹/2% Convertible Subordinated Notes due December 2006 at a purchase price of $975.00 per $1,000 principal amount outstanding. The amount of the filing fee was calculated in accordance with Rule 0-11 of the Securities Exchange Act of 1934, as amended.

o

Check box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the form or schedule and the date of its filing.

o

Check box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

Check the appropriate boxes to designate any transactions to which this statement relates:

o	third party tender offer subject to Rule 14d-1	o	going-private transaction subject toRule 13e-3
ý	issuer tender offer subject to Rule 13e-4	o	amendment to Schedule 13D under Rule 13d-2

Check the following box if the filing is a final amendment reporting the results of the tender offer. o

        This Tender Offer Statement on Schedule TO ("Schedule TO") relates to an offer by Cephalon, Inc., a Delaware corporation (the "Company"), to purchase for cash any and all of the Company's 2¹/2% Convertible Subordinated Notes due December 2006 upon the terms and subject to the conditions contained in the Offer to Purchase dated June 10, 2005 (as amended or supplemented from time to time, the "Offer to Purchase"), a copy of which is attached hereto as Exhibit (a)(1)(i), and the related Letter of Transmittal (as amended or supplemented from time to time, the "Letter of Transmittal"), a copy of which is attached hereto as Exhibit (a)(1)(ii) (which together with the Offer to Purchase constitutes the "Offer").

        This Tender Offer Statement on Schedule TO is intended to satisfy the reporting requirements of Rule 13e-4 under the Securities Exchange Act of 1934, as amended. The information in the Offer to Purchase and the related Letter of Transmittal, copies of which are filed as Exhibit (a)(1)(i) and (a)(1)(ii) hereto, respectively, is incorporated by reference in answer to Items 1 through 11 of this Tender Offer Statement on Schedule TO except as otherwise set forth below.

Item 3. Identity and Background of Filing Person.

        The following table lists the names of all directors and executive officers of the Company. The mailing address of each director and executive officer is: c/o Cephalon, Inc., 41 Moores Road, Frazer, PA 19355.

Name	Office
Frank Baldino, Jr. Ph.D.	Director, Chairman and Chief Executive Officer
William P. Egan	Director
Martyn D. Greenacre	Director
Vaughn M. Kailian	Director
Charles A. Sanders, M.D.	Director
Gail R. Wilensky, Ph.D.	Director
Dennis L. Winger	Director
Horst Witzel, Dr. – Ing.	Director
Paul Blake, F.R.C.P.	Executive Vice President, Worldwide Clinical Research and Regulatory Affairs
J. Kevin Buchi	Senior Vice President and Chief Financial Officer
Peter E. Grebow, Ph.D.	Executive Vice President, Worldwide Technical Operations
John E. Osborn	Senior Vice President, General Counsel and Secretary
Robert P. Roche, Jr.	Executive Vice President, Worldwide Pharmaceutical Operations
Carl A. Savini	Senior Vice President and Chief Administrative Officer
Jeffry L. Vaught, Ph.D.	Senior Vice President and President, Research and Development

Item 5. Past Contacts, Transactions, Negotiations and Agreements.

        (e)   Agreements Involving the Subject Company's Securities.    The Company is a party to the following agreements, arrangements or understandings involve the Company's securities: (i) Amended and Restated Rights Agreement, dated as of January 1, 1999, between the Company and StockTrans, Inc.; (ii) First Amendment to Amended and Restated Rights Agreement, dated July 31, 2000, between the Company and StockTrans, Inc.; (iii) Second Amendment to Amended and Restated Rights Agreement, dated October 27, 2003, between the Company and StockTrans, Inc.; (iv) Indenture, dated as of December 11, 2001, between the Company and State Street Bank and Trust Company relating to the Company's 2¹/2% Convertible Subordinated Notes due December 15, 2006; (v) Indenture, dated as of June 11, 2003, between the Company and U.S. Bank National Association relating to the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2008, and the Company's Zero Coupon Convertible Subordinated Notes due June 15,

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2033, first putable June 15, 2010; (vi) Registration Rights Agreement, dated as of June 11, 2003, between the Company and Credit Suisse First Boston LLC, CIBC World Markets Corp., J.P. Morgan Securities Inc., Morgan Stanley & Co. Incorporated, SG Cowen Securities Corporation, ABN AMRO Rothschild LLC, Citigroup Global Markets Inc. and Lehman Brothers Inc. relating to the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2008, and the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2010; (vii) Indenture, dated as of December 20, 2004, between the Company and U.S. Bank National Association relating to the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2008, and the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2010; (viii) Registration Rights Agreement, dated as of December 20, 2004, between the Company and U.S Bank National Association relating to the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2008, and the Company's Zero Coupon Convertible Subordinated Notes due June 15, 2033, first putable June 15, 2010; (ix) Indenture, dated June 7, 2005, between the Company and U.S. Bank, National Association relating to the Company's 2.00% Convertible Senior Subordinated Notes due 2015; (x) Five Year Convertible Note Hedge, dated June 6, 2003, between the Company and Credit Suisse First Boston International; (xi) Seven Year Convertible Note Hedge, dated June 6, 2003, between the Company and Credit Suisse First Boston International; (xii) Five Year Warrant, dated June 6, 2003, between the Company and Credit Suisse First Boston International; (xiii) Seven Year Warrant, dated June 6, 2003, between the Company and Credit Suisse First Boston International; (xiv) Five Year Convertible Note Hedge, dated December 3, 2004, between the Company and Credit Suisse First Boston International; (xv) Seven Year Convertible Note Hedge, dated December 3, 2004, between the Company and Credit Suisse First Boston International; (xvi) Convertible Note Hedge Confirmation, dated as of June 2, 2005, between the Company and Deutsche Bank AG; and (xvii) Warrant Confirmation, dated as of June 2, 2005, between the Company and Deutsche Bank AG.

        The foregoing agreements, arrangements and understandings are attached hereto as Exhibits (d)(1)—(d)(17) and incorporated herein by reference.

Item 8. Interest in Securities of the Subject Company.

        None.

Item 11. Additional Information.

        None.

Item 12. Exhibits.

Exhibit No.	Description
(a)(1)(i)	Offer to Purchase, dated June 10, 2005.
(a)(1)(ii)	Form of Letter of Transmittal.
(a)(1)(iii)	Form of Letter to Brokers, Dealers, Commercial Banks, Trust Companies and other Nominees.
(a)(1)(iv)	Form of Letter to Clients.
(a)(1)(v)	Form of Notice of Guaranteed Delivery.
(a)(1)(vi)	Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9.
(a)(2)	None.
(a)(3)	None.
(a)(4)	None.
(a)(5)	Press Release, dated June 10, 2005.
(b)	None.

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(d)(1)	Amended and Restated Rights Agreement, dated as of January 1, 1999, between the Company and StockTrans, Inc., included as Exhibit 1 to the Company's Form 8-A/A (12G) filed January 20, 1999, is hereby incorporated by reference.
(d)(2)	First Amendment to Amended and Restated Rights Agreement, dated July 31, 2000, between the Company and StockTrans, Inc., included as Exhibit 1 to the Company's Form 8-A/12G filed on August 2, 2000, is hereby incorporated by reference.
(d)(3)	Second Amendment to Amended and Restated Rights Agreement, dated October 27, 2003, between the Company and StockTrans, Inc., included as Exhibit 1 to the Company's Form 8-A/12G on October 27, 2003, is hereby incorporated by reference.
(d)(4)	Indenture, dated as of December 11, 2001, between the Company and State Street Bank and Trust Company, included as Exhibit 4.1 to the Company's Registration Statement on Form S-3 (Registration No. 333-82788) filed on February 14, 2002, is hereby incorporated by reference.
(d)(5)	Indenture, dated as of June 11, 2003, between the the Company and U.S. Bank National Association, included as Exhibit 4.1 to the Company's Quarterly Report on Form 10-Q for the period ended June 30, 2003, is hereby incorporated by reference.
(d)(6)	Registration Rights Agreement, dated as of June 11, 2003, between the Company and Credit Suisse First Boston LLC, CIBC World Markets Corp., J.P. Morgan Securities Inc., Morgan Stanley & Co. Incorporated, SG Cowen Securities Corporation, ABN AMRO Rothschild LLC, Citigroup Global Markets Inc. and Lehman Brothers Inc., included as Exhibit 10.4 to the Company's Quarterly Report on Form 10-Q for the period ended June 30, 2003, is hereby incorporated by reference.
(d)(7)	Indenture, dated as of December 20, 2004, between the Company and U.S. Bank National Association, included as Exhibit 4.l to the Company's Current Report on Form 8-K dated December 21, 2004, is hereby incorporated by reference.
(d)(8)	Registration Rights Agreement, dated as of December 20, 2004, between the Company and U.S Bank National Association, included as Exhibit 4.2 to the Company's Current Report on Form 8-K dated December 21, 2004, is hereby incorporated by reference.
(d)(9)	Indenture, dated June 7, 2005, between the Company and U.S. Bank, National Association, included as Exhibit 4.1 to the Company's Current Report on Form 8-K dated June 8, 2005, is hereby incorporated by reference.
(d)(10)	Five Year Convertible Note Hedge, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(1) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(11)	Seven Year Convertible Note Hedge, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(2) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(12)	Five Year Warrant, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(3) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(13)	Seven Year Warrant, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(4) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(14)	Five Year Convertible Note Hedge, dated December 3, 2004, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(5) to Amendment No. 1 to the Company's Tender Offer Statement on Schedule TO filed December 14, 2004, is hereby incorporated by reference.

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(d)(15)	Seven Year Convertible Note Hedge, dated December 3, 2004, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(5) to Amendment No. 1 to the Company's Tender Offer Statement on Schedule TO filed December 14, 2004, is hereby incorporated by reference.
(d)(16)	Convertible Note Hedge Confirmation, dated as of June 2, 2005, between the Company and Deutsche Bank AG, included as Exhibit 10.1 to the Company's Current Report on Form 8-K filed June 8, 2005, is hereby incorporated by reference.
(d)(17)	Warrant Confirmation, dated as of June 2, 2005, between the Company and Deutsche Bank AG, included as Exhibit 10.2 to the Company's Current Report on Form 8-K filed June 8, 2005, is hereby incorporated by reference.
(g)	None.
(h)	None.

Item 13. Information required by Schedule 13E-3.

        Not applicable.

5

SIGNATURE

        After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Cephalon, Inc.
By:	/s/  J. KEVIN BUCHI
	Name:	J. Kevin Buchi
	Title:	Senior Vice President and Chief Financial Officer

Dated: June 10, 2005

6

Exhibit Index

Exhibit No.	Description
(a)(1)(i)	Offer to Purchase, dated June 10, 2005.
(a)(1)(ii)	Form of Letter of Transmittal.
(a)(1)(iii)	Form of Letter to Brokers, Dealers, Commercial Banks, Trust Companies and other Nominees.
(a)(1)(iv)	Form of Letter to Clients.
(a)(1)(v)	Form of Notice of Guaranteed Delivery.
(a)(1)(vi)	Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9.
(a)(2)	None.
(a)(3)	None.
(a)(4)	None.
(a)(5)	Press Release, dated June 10, 2005.
(b)	None.
(d)(1)	Amended and Restated Rights Agreement, dated as of January 1, 1999, between the Company and StockTrans, Inc., included as Exhibit 1 to the Company's Form 8-A/A (12G) filed January 20, 1999, is hereby incorporated by reference.
(d)(2)	First Amendment to Amended and Restated Rights Agreement, dated July 31, 2000, between the Company and StockTrans, Inc., included as Exhibit 1 to the Company's Form 8-A/12G filed on August 2, 2000, is hereby incorporated by reference.
(d)(3)	Second Amendment to Amended and Restated Rights Agreement, dated October 27, 2003, between the Company and StockTrans, Inc., included as Exhibit 1 to the Company's Form 8-A/12G on October 27, 2003, is hereby incorporated by reference.
(d)(4)	Indenture, dated as of December 11, 2001, between the Company and State Street Bank and Trust Company, included as Exhibit 4.1 to the Company's Registration Statement on Form S-3 (Registration No. 333-82788) filed on February 14, 2002, is hereby incorporated by reference.
(d)(5)	Indenture, dated as of June 11, 2003, between the the Company and U.S. Bank National Association, included as Exhibit 4.1 to the Company's Quarterly Report on Form 10-Q for the period ended June 30, 2003, is hereby incorporated by reference.
(d)(6)	Registration Rights Agreement, dated as of June 11, 2003, between the Company and Credit Suisse First Boston LLC, CIBC World Markets Corp., J.P. Morgan Securities Inc., Morgan Stanley & Co. Incorporated, SG Cowen Securities Corporation, ABN AMRO Rothschild LLC, Citigroup Global Markets Inc. and Lehman Brothers Inc., included as Exhibit 10.4 to the Company's Quarterly Report on Form 10-Q for the period ended June 30, 2003, is hereby incorporated by reference.
(d)(7)	Indenture, dated as of December 20, 2004, between the Company and U.S. Bank National Association, included as Exhibit 4.l to the Company's Current Report on Form 8-K dated December 21, 2004, is hereby incorporated by reference.
(d)(8)	Registration Rights Agreement, dated as of December 20, 2004, between the Company and U.S Bank National Association, included as Exhibit 4.2 to the Company's Current Report on Form 8-K dated December 21, 2004, is hereby incorporated by reference.
(d)(9)	Indenture, dated June 7, 2005, between the Company and U.S. Bank, National Association, included as Exhibit 4.1 to the Company's Current Report on Form 8-K dated June 8, 2005, is hereby incorporated by reference.
(d)(10)	Five Year Convertible Note Hedge, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(1) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(11)	Seven Year Convertible Note Hedge, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(2) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.

(d)(12)	Five Year Warrant, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(3) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(13)	Seven Year Warrant, dated June 6, 2003, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(4) to the Company's Tender Offer Statement on Schedule TO filed November 16, 2004, is hereby incorporated by reference.
(d)(14)	Five Year Convertible Note Hedge, dated December 3, 2004, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(5) to Amendment No. 1 to the Company's Tender Offer Statement on Schedule TO filed December 14, 2004, is hereby incorporated by reference.
(d)(15)	Seven Year Convertible Note Hedge, dated December 3, 2004, between the Company and Credit Suisse First Boston International, included as Exhibit (d)(5) to Amendment No. 1 to the Company's Tender Offer Statement on Schedule TO filed December 14, 2004, is hereby incorporated by reference.
(d)(16)	Convertible Note Hedge Confirmation, dated as of June 2, 2005, between the Company and Deutsche Bank AG, included as Exhibit 10.1 to the Company's Current Report on Form 8-K filed June 8, 2005, is hereby incorporated by reference.
(d)(17)	Warrant Confirmation, dated as of June 2, 2005, between the Company and Deutsche Bank AG, included as Exhibit 10.2 to the Company's Current Report on Form 8-K filed June 8, 2005, is hereby incorporated by reference.
(g)	None.
(h)	None.

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Item 3. Identity and Background of Filing Person.
Item 5. Past Contacts, Transactions, Negotiations and Agreements.
Item 8. Interest in Securities of the Subject Company.
Item 11. Additional Information.
Item 12. Exhibits.
Item 13. Information required by Schedule 13E-3.

SIGNATURE
Exhibit Index

EX-99.A1I

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Exhibit 99(a)(1)(i)

OFFER TO PURCHASE

CEPHALON, INC.

Offer to Purchase for Cash any and all of the Outstanding
2¹/2% Convertible Subordinated Notes Due December 2006
(CUSIP Nos. 156708AD1 and 156708AE9) of Cephalon, Inc.

Cephalon, Inc., a Delaware corporation, is offering to purchase for cash, upon the terms and subject to the conditions set forth in this Offer to Purchase (as it may be amended or supplemented from time to time, this "Statement") and the accompanying Letter of Transmittal (as it may be amended or supplemented from time to time, the "Letter of Transmittal"), any and all of its outstanding 2¹/2% Convertible Subordinated Notes due December 2006 (the "Notes") from each holder of Notes. The offer, on the terms set forth in this Statement and the Letter of Transmittal, is referred to as the "Offer."

Subject to the terms and conditions of the Offer, holders who properly tender their Notes at or prior to the Expiration Date, defined below, will receive $975.00 for each $1,000 principal amount of Notes purchased pursuant to the Offer, plus accrued and unpaid interest up to, but not including, the date of payment for the Notes accepted for payment.

This Statement and the accompanying Letter of Transmittal contain or incorporate by reference important information that should be read before any decision is made with respect to the Offer. Please see the section titled "Summary" beginning on page 1 of this Statement for a more complete description of the terms of the Offer.

See "Certain Significant Considerations" beginning on page 16 for a discussion of certain factors that should be considered in evaluating the Offer.

As of June 9, 2005, there was $521,750,000 aggregate principal amount of Notes outstanding. The Notes are convertible into shares of our common stock at a conversion rate (subject to adjustment) of 12.3458 shares per $1,000 principal amount of Notes, which is equal to a conversion price of $81.00 per share. The Notes are eligible for trading on The Portal Market. However, there is no existing public market for the Notes and we believe that trading in the Notes has been limited and sporadic. Our common stock currently is quoted on The Nasdaq National Market under the symbol "CEPH." The last reported sale price of our common stock on June 9, 2005 was $39.07 per share.

THE OFFER AND YOUR WITHDRAWAL RIGHTS WILL EXPIRE AT 5 P.M., NEW YORK CITY TIME, ON      
JULY 11, 2005, UNLESS EXTENDED (SUCH DATE AND TIME, AS IT MAY BE EXTENDED, THE "EXPIRATION DATE").

OUR BOARD OF DIRECTORS HAS APPROVED THE OFFER. HOWEVER, NEITHER WE NOR ANY MEMBER OF OUR BOARD OF DIRECTORS, THE DEALER MANAGER FOR THE OFFER, THE DEPOSITARY FOR THE OFFER OR THE INFORMATION AGENT FOR THE OFFER MAKES ANY RECOMMENDATION TO YOU AS TO WHETHER YOU SHOULD TENDER OR REFRAIN FROM TENDERING YOUR NOTES. YOU MUST MAKE YOUR OWN DECISION AS TO WHETHER TO TENDER YOUR NOTES, AND, IF SO, HOW MANY NOTES TO TENDER. IN DOING SO, YOU SHOULD READ CAREFULLY THE INFORMATION IN THIS STATEMENT AND IN THE RELATED LETTER OF TRANSMITTAL, INCLUDING OUR REASONS FOR MAKING THE OFFER. YOU SHOULD DISCUSS WHETHER TO TENDER YOUR NOTES WITH YOUR BROKER OR OTHER FINANCIAL ADVISORS AND TAX ADVISORS.

The Offer is not conditioned on any minimum principal amount of Notes being tendered. However, the Offer is subject to the satisfaction of certain conditions. See the section titled "Terms of the Offer—Conditions to the Offer" beginning on page 13 of this Statement.

Questions and requests for assistance may be directed to Deutsche Bank Securities Inc. (the "Dealer Manager") or Morrow & Co., Inc. (the "Information Agent"). Requests for additional copies of this Statement or the Letter of Transmittal should be directed to the Information Agent.

The Dealer Manager for the Offer is:

Deutsche Bank Securities

June 10, 2005

TABLE OF CONTENTS

SUMMARY	1
CEPHALON	6
PURPOSE OF THE OFFER	6
SOURCE AND AMOUNT OF FUNDS	7
PRICE RANGE OF COMMON STOCK	7
DIVIDEND POLICY	7
SUMMARY FINANCIAL INFORMATION	8
TERMS OF THE OFFER	9
CERTAIN SIGNIFICANT CONSIDERATIONS	16
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS	17
WHERE YOU CAN FIND MORE INFORMATION	18
DOCUMENTS INCORPORATED BY REFERENCE	18
CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES	19
THE DEALER MANAGER, DEPOSITARY AND INFORMATION AGENT	23
MISCELLANEOUS	24

        THE OFFER IS BEING MADE TO ALL HOLDERS OF NOTES. WE ARE NOT AWARE OF ANY JURISDICTION IN WHICH THE MAKING OF THE OFFER IS PROHIBITED BY ADMINISTRATIVE OR JUDICIAL ACTION PURSUANT TO A STATE STATUTE. IF WE BECOME AWARE OF ANY JURISDICTION WHERE THE MAKING OF THE OFFER IS SO PROHIBITED, WE WILL MAKE A GOOD FAITH EFFORT TO COMPLY WITH ANY SUCH STATUTE. IF, AFTER SUCH GOOD FAITH EFFORT, WE CANNOT COMPLY WITH ANY APPLICABLE STATUTE, THE OFFER WILL NOT BE MADE TO (NOR WILL TENDERS BE ACCEPTED FROM OR ON BEHALF OF) THE HOLDERS IN SUCH JURISDICTION.

IMPORTANT

        No person has been authorized to give any information or to make any representations in connection with the Offer other than those contained in this Statement and the related Letter of Transmittal, and, if given or made, such information or representations should not be relied upon as having been authorized by Cephalon, the Dealer Manager, the Depositary (as defined herein) or the Information Agent. This Statement and the related documents do not constitute an offer to buy or solicitation of an offer to sell Notes in any circumstances in which such offer or solicitation is unlawful. In those jurisdictions where the securities, "blue sky" or other laws require the Offer to be made by a licensed broker or dealer, the Offer will be deemed to be made on behalf of Cephalon by the Dealer Manager or one or more registered brokers or dealers licensed under the laws of such jurisdiction. Neither the delivery of this Statement and related documents nor any purchase of Notes will, under any circumstances, create any implication that the information contained in this Statement or such other documents is current as of any time after the date of any such document.

        We and our affiliates, including our executive officers and directors, will be prohibited by Rule 13e-4 under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), from purchasing any of the Notes outside of the Offer until at least the tenth business day after the expiration or termination of the Offer. Following that time, we expressly reserve the absolute right, in our sole discretion from time to time in the future, to purchase any of the Notes, whether or not any Notes are purchased pursuant to the Offer, through open market purchases, privately negotiated transactions, tender offers, exchange offers or otherwise, upon such terms and at such prices as we may determine, which may be more or less than the price to be paid pursuant to the Offer and could be for cash or other consideration. We cannot assure you as to which, if any, of these alternatives, or combinations thereof, we will pursue.

        The CUSIP numbers referenced in this Statement have been assigned by Standard & Poor's Corporation and are included solely for the convenience of the holders. None of Cephalon, the Dealer Manager, the Depositary or the Information Agent is responsible for the selection or use of the above CUSIP numbers, and no representation is made as to their correctness on the Notes or as indicated in this Statement, the Letter of Transmittal or any other document.

i

SUMMARY

        The following are answers to some of the questions that you, as a holder, may have about the Offer. We urge you to read carefully the remainder of this Statement, the Letter of Transmittal and the documents that are incorporated in this Statement by reference because the information in this summary is not complete and such documents contain important information.

INFORMATION ABOUT THE OFFER

WHO IS OFFERING TO PURCHASE THE NOTES?

•

Cephalon, Inc., a Delaware corporation, is offering to purchase the Notes.

WHAT CLASS OF SECURITIES IS SOUGHT IN THE OFFER?

•

We are offering to purchase any and all of our outstanding 2¹/2% Convertible Subordinated Notes due December 2006, which we refer to as the "Notes." The Notes were issued pursuant to an Indenture, which we refer to as the "Indenture," dated as of December 11, 2001, between us and U.S. Bank National Association, which we refer to as the "Trustee."

WHY IS CEPHALON MAKING THE OFFER?

•

We are making the Offer in order to acquire any or all of the outstanding Notes. Purchasing the Notes will reduce our outstanding debt and reduce our interest expense. You should read the section titled "Purpose of the Offer" beginning on page 6 for more information.

WHAT WILL CEPHALON DO WITH THE NOTES PURCHASED?

•

We will deliver the Notes that we purchase in the Offer to the Trustee for cancellation and those Notes will cease to be outstanding.

HOW MUCH IS CEPHALON OFFERING TO PAY FOR THE NOTES?

•

We are offering to pay $975.00 in cash plus accrued and unpaid interest to, but not including, the payment date for each $1,000 principal amount of Notes.

ARE THERE CONDITIONS TO THE OFFER?

•

The Offer is not conditioned on a minimum principal amount of Notes being tendered. However, we may terminate or amend the Offer or may, subject to applicable law, postpone the acceptance for payment of, or the purchase of and payment for, Notes tendered upon the occurrence of certain events, including material litigation, government investigations, national crisis or other events adversely affecting our business or the general markets. You should read the section titled "Terms of the Offer—Conditions to the Offer" beginning on page 13 for more information.

HOW MANY NOTES WILL CEPHALON PURCHASE?

•

We will purchase for cash, upon the terms and subject to the conditions of the Offer, any and all of the Notes that are validly tendered and not properly withdrawn.

DOES CEPHALON HAVE THE FINANCIAL RESOURCES TO PURCHASE THE NOTES?

•

Yes. On June 7, 2005, we issued and sold $800,000,000 million in aggregate principal amount of our 2.00% Convertible Senior Subordinated Notes due June 1, 2015. A portion of the net proceeds of the sale of these notes will be used to purchase the Notes. You should read the section titled "Source and Amount of Funds" on page 7 for more information.

1

WHAT IS THE MARKET VALUE OF THE NOTES?

•

The Notes are eligible for trading on The Portal Market, a subsidiary of The Nasdaq Stock Market, Inc. We believe that trading in the Notes has been limited and sporadic. The Notes are convertible into shares of our common stock at a conversion rate (subject to adjustment) of 12.3458 shares per $1,000 principal amount of Notes. Our common stock is quoted on The Nasdaq National Market under the symbol "CEPH." On June 9, 2005, the high and low sale prices per share for our common stock as reported on The Nasdaq National Market were $39.60 and $38.14, respectively. You should read the sections titled "Price Range of Common Stock" on page 7 and "Certain Significant Considerations—Limited Trading Market" on page 16 for more information.

HOW DO I TENDER NOTES?

•

There are three ways to tender your Notes, depending upon the manner in which your Notes are held:



•

If your Notes are registered in your name,



•

complete and sign the Letter of Transmittal or a facsimile copy in accordance with the instructions in the Letter of Transmittal,



•

mail or deliver it and any other required documents to U.S. Bank National Association, which we refer to as the "Depositary," at the address set forth on the back cover of this Statement, and



•

either deliver the certificates for the tendered Notes to the Depositary or transfer your Notes pursuant to the book-entry transfer procedures described in this Statement.



•

If your Notes are registered in the name of a broker, dealer, commercial bank, trust company or other nominee, meaning your Notes are owned in "street name," then you must instruct your broker, dealer, commercial bank, trust company or other nominee to tender your Notes.



•

If your Notes are held of record by The Depository Trust Company, or DTC, you may tender them through DTC's Automated Tender Offer Program.

You should read the section titled "Terms of the Offer—Procedure for Tendering Notes" beginning on page 10 for more information on how to tender your Notes.

If you want to tender your Notes but:

•

your certificates for your Notes are not immediately available or cannot be delivered to the Depositary on or before the Expiration Date, or



•

you cannot comply with the procedure for book-entry transfer on or before the Expiration Date, or



•

your other required documents cannot be delivered to the Depositary on or before the Expiration Date,

you can still tender your Notes if you comply with the guaranteed delivery procedure described in "Terms of the Offer—Procedure for Tendering Notes—Guaranteed Delivery" beginning on page 11.

HOW LONG DO I HAVE TO TENDER NOTES?

•

The Offer expires at 5:00 p.m., New York City time, on July 11, 2005, unless we extend the Offer in our sole discretion.

2

MAY THE OFFER BE EXTENDED, AMENDED OR TERMINATED AND UNDER WHAT CIRCUMSTANCES?

•

We may extend the Offer until the conditions to the completion of the Offer described in the section titled "Terms of the Offer—Conditions to the Offer" beginning on page 13 are satisfied. We may amend the Offer in any respect by giving written notice of such amendment to the Depositary.

We may extend or amend the Offer in our sole discretion. If we extend the Offer, we will delay the acceptance of any Notes that have been tendered. We may terminate the Offer under certain circumstances. You should read the section titled "Terms of the Offer—Extension, Waiver, Amendment and Termination" on page 15 for more information.

HOW WILL HOLDERS OF NOTES BE NOTIFIED IF THE OFFER IS EXTENDED?

•

If we extend the Offer, we will notify you as promptly as practicable by a public announcement, which will be issued no later than 9:00 a.m., New York City time, on the first business day after the previously scheduled Expiration Date. Without limiting the manner in which we may choose to make any public announcement, we have no obligation to publish, advertise or otherwise communicate any public announcement other than by issuing a press release to the Dow Jones News Service. You should read the section titled "Terms of the Offer—Extension, Waiver, Amendment and Termination" on page 15 for more information.

In addition, if we materially change the terms of the Offer or the information concerning the Offer, or if we waive a material condition of the Offer, we will disseminate additional tender offer materials and extend the Offer to the extent required by Rule 13e-4(d)(2) and Rule 13e-4(e)(3) under the Exchange Act.

WHEN WILL HOLDERS RECEIVE PAYMENT FOR TENDERED NOTES?

•

You will receive payment for your Notes promptly after the date on which we accept all Notes properly tendered and not validly withdrawn. The timing of our acceptance for payment of Notes tendered pursuant to the Offer is subject to Rule 13e-4(f)(5) under the Exchange Act, which requires that we pay the consideration offered or return the Notes deposited by or on behalf of holders promptly after the termination or withdrawal of the Offer. You should read the section titled "Terms of the Offer—General" beginning on page 9 for more information.

CAN HOLDERS WITHDRAW TENDERED NOTES?

•

You may withdraw your tendered Notes at any time on or before the Offer expires at 5:00 p.m., New York City time, on July 11, 2005 or, if the Offer is extended, the time and date when the extended Offer expires. You may also withdraw your Notes if we have not accepted them for payment by August 5, 2005.

HOW DO HOLDERS WITHDRAW PREVIOUSLY TENDERED NOTES?

•

To withdraw your previously tendered Notes, you must deliver a written or facsimile transmission notice of withdrawal with the required information to the Depositary before your right to withdraw has expired. You may not rescind a withdrawal of tendered Notes. However, you may re-tender your Notes by again following the proper tender procedures. You should read the section titled "Terms of the Offer—Withdrawal of Tendered Notes" beginning on page 12 for more information on how to withdraw previously tendered Notes.

3

WHAT HAPPENS TO NOTES THAT ARE NOT TENDERED?

•

Any Notes that remain outstanding after the completion of the Offer will continue to be our obligations. Holders of those outstanding Notes will continue to have all the rights associated with those Notes. You should read the section titled "Certain Significant Considerations" beginning on page 16.

MAY HOLDERS STILL CONVERT NOTES INTO SHARES OF CEPHALON COMMON STOCK?

•

Yes. However, if you tender your Notes in the Offer, you may convert your Notes only if you properly withdraw your Notes before your right to withdraw has expired. The Notes are convertible into shares of our common stock at a conversion rate (subject to adjustment) of 12.3458 shares of our common stock for each $1,000 principal amount of Notes, which is equal to a conversion price of $81.00 per share.

WHAT ARE THE TAX CONSEQUENCES TO HOLDERS IF THEY TENDER THEIR NOTES?

•

Holders should consult their own tax advisors regarding the federal, state, local and foreign income, franchise, personal property and any other tax consequences of the tendering of the Notes pursuant to the Offer. A U.S. Holder (as defined below in the section titled "Certain United States Federal Income Tax Consequences") who sells Notes to us pursuant to the Offer will generally recognize gain or loss in an amount equal to the difference between the amount received in exchange for the Notes (other than amounts attributable to accrued but unpaid interest) and such U.S. Holder's adjusted tax basis in the Notes sold. See "Certain United States Federal Income Tax Consequences—U.S. Holders" beginning on page 20. Non-U.S. Holders (as defined below in the section titled "Certain United States Federal Income Tax Consequences") should refer to "Certain United States Federal Income Tax Consequences—Non-US. Holders" beginning on page 21 for a discussion of certain U.S. federal income tax consequences applicable to Non-U.S. Holders.

DO HOLDERS HAVE TO PAY A BROKERAGE COMMISSION FOR TENDERING NOTES?

•

No brokerage commissions are payable by holders to us, the Dealer Manager, the Depositary or the Information Agent in connection with the tender of their Notes in the Offer. If your Notes are held by a nominee, such nominee may charge you a transaction amount. Except as set forth in Instruction 7 of the Letter of Transmittal, we will pay any transfer taxes with respect to the transfer and sale of Notes pursuant to the Offer.

WHERE CAN HOLDERS GET MORE INFORMATION REGARDING THE OFFER?

•

If you have any questions or requests for assistance or for additional copies of this Statement or the Letter of Transmittal, please contact Morrow & Co., Inc., the Information Agent for the Offer, at (800) 654-2468 or (800) 607-0088. You may also contact Deutsche Bank Securities Inc., the Dealer Manager for the Offer, at (212) 250-5600. Beneficial owners may also contact their broker, dealer, commercial bank, trust company or other nominee through which they hold their Notes with questions and requests for assistance.

IS CEPHALON MAKING ANY RECOMMENDATION ABOUT THE OFFER?

•

Neither we nor the Dealer Manager, the Depositary or the Information Agent makes any recommendation as to whether or not you should tender your Notes pursuant to the Offer. Holders should determine whether or not to tender their Notes pursuant to the Offer based upon, among other things, their own assessment of the current market value of the Notes, liquidity needs and investment objectives.

4

In this Statement, "Cephalon, Inc.," "Cephalon," "we," "us," "our" and "the Company" refer to Cephalon, Inc. and do not include its subsidiaries, unless the context requires otherwise, and except that such references in the sections of this Statement titled "Cephalon," "Summary Financial Information" and "Special Note Regarding Forward-Looking Statements" are to Cephalon, Inc. and its consolidated subsidiaries.

5

CEPHALON

        We are an international biopharmaceutical company dedicated to the discovery, development and marketing of products to treat sleep disorders, neurological and psychiatric disorders, cancer and pain. In addition to conducting an active research and development program, we market three products in the United States and a number of products in various countries throughout Europe.

        Our corporate headquarters are in Frazer, Pennsylvania and our research and development headquarters are in West Chester, Pennsylvania. We also have offices in Utah, Minnesota, France, the United Kingdom, Germany and Switzerland. We operate manufacturing facilities in France for the production of modafinil, which is the active drug substance in PROVIGIL® (modafinil) tablets [C-IV]. In addition, we operate manufacturing facilities in Salt Lake City, Utah for the production of ACTIQ® (oral transmucosal fentanyl citrate) [C-II] for distribution and sale in the European Union and the United States and Eden Prairie and Brooklyn Park, Minnesota, for the production of orally disintegrating versions of drugs for pharmaceutical company partners.

        Our three biggest products in terms of product sales, PROVIGIL, ACTIQ and GABITRIL® (tiagabine hydrochloride), comprised approximately 90% and 86% of our total worldwide net product sales for the fiscal year ended December 31, 2004 and the quarter ended March 31, 2005, respectively. The majority of PROVIGIL, ACTIQ and GABITRIL sales are in the U.S. market. Outside the United States, our commercial activities are concentrated primarily in France, the United Kingdom and Germany.

        In addition to clinical programs focused on our marketed products, we have significant research programs that seek to discover and develop therapeutics to treat neurological and oncological disorders. Our technology principally focuses on an understanding of kinases and the role they play in cellular survival and proliferation. We have coupled this knowledge with a library of small, synthetic molecule inhibitors of kinases that allows us to intervene in these processes. This technology base has resulted in two molecules that are currently in clinical development. We have a program with a lead molecule, CEP-701, and are currently conducting Phase 2 clinical trials in patients suffering from prostate cancer and acute myeloid leukemia. We are also conducting a Phase 1/2 clinical study with another molecule, CEP-7055, to evaluate safety and tolerability and to gather preliminary evidence of efficacy in patients with treatment refractory tumors. As part of our corporate strategy, we often seek to share the risk of our research and development activities with corporate partners and, to that end, we have entered into agreements to share the costs of developing and/or commercializing certain of these molecules.

        We are a Delaware corporation with our principal executive offices located at 41 Moores Road, Frazer, Pennsylvania 19355. Our telephone number is (610) 344-0200 and our web site address is www.cephalon.com. We include our web site address in this document only as an inactive textual reference and do not intend it to be an active link to our web site. Accordingly, information contained in our web site is not incorporated by reference in, and should not be considered a part of, this Statement.

PURPOSE OF THE OFFER

        We are making the Offer in order to acquire any or all of the outstanding Notes. Purchasing the Notes will reduce our outstanding debt and reduce our interest expense. We will deliver the Notes that we purchase in the Offer to the Trustee for cancellation, and those Notes will cease to be outstanding. Any Notes that remain outstanding after the Offer will continue to be our obligations. Holders of those outstanding Notes will continue to have all

6

the rights associated with those Notes. We are not seeking the approval of holders for any amendment to the Notes or the Indenture.

SOURCE AND AMOUNT OF FUNDS

        On June 7, 2005, we issued and sold $800 million in aggregate principal amount of our 2.00% Convertible Senior Subordinated Notes due June 1, 2015. We will use a portion of the net proceeds of the sale of these notes to purchase the Notes accepted for payment pursuant to the Offer.

PRICE RANGE OF COMMON STOCK

        The Notes are eligible for trading on The Portal Market. However, there is no existing public market for the Notes and we believe that trading in the Notes has been limited and sporadic. The Notes are convertible into shares of our common stock at a conversion rate (subject to adjustment) of 12.3458 shares per $1,000 principal amount of Notes. Our common stock currently is quoted on The Nasdaq National Market under the symbol "CEPH." The last reported sale price of our common stock on June 9, 2005 was $39.07 per share.

        The following table sets forth, for the periods indicated below, the range of high and low sale prices per share for our common stock as reported on The Nasdaq National Market.

		High		Low
2003
	First Quarter	$	54.95	$	39.82
	Second Quarter		48.70		36.91
	Third Quarter		50.71		40.27
	Fourth Quarter		49.46		44.29
2004
	First Quarter	$	60.98	$	48.10
	Second Quarter		60.10		50.54
	Third Quarter		54.96		41.58
	Fourth Quarter		51.73		44.68
2005
	First Quarter	$	52.24	$	45.44
	Second Quarter (through June 9, 2005)		47.54		38.14

DIVIDEND POLICY

        We have not paid any dividends on our common stock since our inception and do not anticipate paying any dividends on our common stock in the foreseeable future.

7

SUMMARY FINANCIAL INFORMATION

Summary financial information

        Set forth below is certain summary historical consolidated financial information with respect to Cephalon derived from financial information contained in our Annual Report on Form 10-K for the year ended December 31, 2004 and our Quarterly Report on Form 10-Q for the quarter ended March 31, 2005. More comprehensive financial information is included in these reports and other documents we file with the U.S. Securities and Exchange Commission (the "SEC"), and the following summary is qualifed in its entirety by reference to these reports and other documents and all the financial information, including any related notes, contained in those documents. See the section titled "Documents Incorporated by Reference."

	Year ended December 31,								Three months ended March 31,
	2004			2003		2002			2005		2004
	(In thousands, except per share data)
Statement of operations data:
Sales	$	980,375		$	685,250	$	465,943		$	266,609	$	210,391
Other revenues		35,050			29,557		40,954			13,372		4,591
Total revenues		1,015,425			714,807		506,897			279,981		214,982
Impairment charge		30,071			—		—			—		—
Acquired in-process research and development		185,700			—		—			—		—
Debt exchange expense		28,230			—		—			—		961
Income tax expense (benefit), net		45,629			46,456		(112,629	)		15,203		12,742
Income (loss) before cumulative effect of a change in accounting principle	$	(73,813	)	$	83,858	$	175,062		$	26,662	$	21,711
Cumulative effect of a change in accounting principle		—			—		(3,534	)		—		—
Net income (loss)	$	(73,813	)	$	83,858	$	171,528		$	26,662	$	21,711
* Basic income (loss) per common share:
Income (loss) before cumulative effect of a change in accounting principle	$	(1.31	)	$	1.49	$	3.14		$	0.46	$	0.38
Cumulative effect of a change in accounting principle		—			—		(.06	)		—		—
	$	(1.31	)	$	1.49	$	3.08		$	0.46	$	0.38
* Diluted income (loss) per common share:
Income (loss) before cumulative effect of a change in accounting principle	$	(1.31	)	$	1.42	$	2.82		$	0.44	$	0.36
Cumulative effect of a change in accounting principle		—			—		(.05	)		—		—
	$	(1.31	)	$	1.42	$	2.77		$	0.44	$	0.36
Weighted average number of shares outstanding		56,489			55,560		55,104			57,994		55,905
Weighted average number of shares outstanding—assuming dilution		56,489			64,076		66,856			65,065		65,006

*

Includes effect from application of EITF 03-6 and 04-8

	As of December 31,
										As of March 31, 2005
	2004			2003			2002
	(In thousands)
Balance sheet data:
Cash, cash equivalents and investments	$	791,676		$	1,155,163		$	582,688		$	810,035
Total assets		2,451,903			2,381,656			1,689,090			2,466,961
Long-term debt		1,284,410			1,409,417			860,897			1,282,652
Accumulated deficit		(395,118	)		(321,305	)		(405,163	)		(368,456	)
Stockholders' equity		830,044			770,370			642,585			842,541

8

TERMS OF THE OFFER

General

        Upon the terms and subject to the conditions set forth in this Statement and in the related Letter of Transmittal, including, if the Offer is extended or amended, the terms and conditions of the extension or amendment, we are offering to purchase for cash any and all of the outstanding Notes at a purchase price of $975.00 for each $1,000 principal amount of Notes plus accrued and unpaid interest to, but excluding, the payment date. You will not be required to pay a commission to the Depositary or the Information Agent in connection with the tender of your Notes in the Offer. If your Notes are held by a nominee, you should consult that nominee to determine whether it will charge any service fee in connection with the Offer. Except as set forth in Instruction 7 of the Letter of Transmittal, we will pay or cause to be paid any transfer taxes with respect to the transfer and sale of Notes pursuant to the Offer.

        We expressly reserve the right, but will not be obligated, to:

•

terminate the Offer and not accept for payment and purchase the tendered Notes and promptly return all tendered Notes to tendering holders, subject to the conditions set forth below;



•

waive any or all of the unsatisfied conditions and accept for payment and purchase all Notes that are validly tendered on or before the Expiration Date and not validly withdrawn;



•

extend the Expiration Date at any time; or



•

amend the Offer.

        Our right to delay acceptance for payment of Notes tendered pursuant to the Offer or the payment for Notes accepted for purchase is subject to Rule 13e-4(f)(5) under the Exchange Act, which requires that we pay the consideration offered or return the Notes deposited by or on behalf of the holders promptly after the termination or withdrawal of the Offer. The Offer will expire at 5:00 p.m., New York City time, on July 11, 2005, unless we extend it in our sole discretion. You should read the sections titled "—Conditions to the Offer" and "—Extension, Waiver, Amendment and Termination" below.

        For purposes of the Offer, we will be deemed to have accepted for payment (and thereby purchased) Notes validly tendered and not properly withdrawn if, as and when we give written notice to the Depositary of our acceptance for payment of such Notes. We will deposit the aggregate purchase price for the Notes accepted for purchase in the Offer with the Depositary, which will act as agent for the tendering holders for the purpose of transmitting payments to the tendering holders. Notes accepted for purchase pursuant to the Offer will be paid for in immediately available funds promptly after the date on which we accept all Notes properly tendered and not withdrawn. Under no circumstances will any interest be paid or payable because of any delay in the transmission of funds by the Depositary.

        We reserve the right to transfer or assign, from time to time, in whole or in part, to one or more of our affiliates the right to purchase any or all of the Notes validly tendered pursuant to the Offer. If this transfer or assignment occurs, the assignee-affiliate will purchase the Notes validly tendered. However, the transfer or assignment will not relieve us of our obligations under the Offer and will not prejudice holders' rights to receive the purchase price in exchange for the Notes validly tendered and accepted for payment.

        NONE OF CEPHALON, THE DEALER MANAGER, THE DEPOSITARY OR THE INFORMATION AGENT MAKES ANY RECOMMENDATION AS TO WHETHER OR NOT HOLDERS SHOULD TENDER THEIR NOTES PURSUANT TO THE OFFER.

9

        There are three ways to tender your Notes, depending on the manner in which your Notes are held:

•

If your Notes are registered in your name,



•

complete and sign the Letter of Transmittal or a facsimile copy in accordance with the instructions in the Letter of Transmittal,



•

mail or deliver it and any other required documents to the Depositary at the address set forth on the back cover of this Statement and



•

either deliver the certificates for the tendered Notes to the Depositary or transfer your Notes pursuant to the book-entry transfer procedures described in this Statement.



•

If your Notes are registered in the name of a broker, dealer, commercial bank, trust company or other nominee, meaning your Notes are owned in "street name," then you must instruct your broker, dealer, commercial bank, trust company or other nominee to tender your Notes; or



•

If your Notes are held of record by DTC, you may tender them through DTC's Automated Tender Offer Program.

        If you want to tender your Notes but:

•

your certificates for your Notes are not immediately available or cannot be delivered to the Depositary on or prior to the Expiration Date, or



•

you cannot comply with the procedure for book-entry transfer on or prior to the Expiration Date, or



•

your other required documents cannot be delivered to the Depositary on or prior to the Expiration Date,

you can still tender your Notes if you comply with the guaranteed delivery procedure described below.

        A HOLDER WITH NOTES REGISTERED IN THE NAME OF A BROKER, DEALER, COMMERCIAL BANK, TRUST COMPANY OR OTHER NOMINEE MUST CONTACT AND INSTRUCT THAT BROKER, DEALER, COMMERCIAL BANK, TRUST COMPANY OR OTHER NOMINEE IF SUCH HOLDER DESIRES TO TENDER THOSE NOTES. TO BE VALID, TENDERS MUST BE RECEIVED BY THE DEPOSITARY ON OR BEFORE THE EXPIRATION DATE.

Procedure for Tendering Notes

        Valid Tender.    For a holder to validly tender Notes pursuant to the Offer, a properly completed and duly executed Letter of Transmittal or facsimile thereof, with any required signature guarantee, or in the case of a book-entry transfer, an Agent's Message (as defined below) in lieu of the Letter of Transmittal, and any other required documents, must be received by the Depositary at the address set forth on the back cover of this Statement on or before the Expiration Date. In addition, on or before the Expiration Date, either:

•

certificates for tendered Notes must be received by the Depositary at such address; or



•

such Notes must be transferred pursuant to the procedures for book-entry transfer, and a confirmation of such tender must be received by the Depositary, including an Agent's Message if the tendering holder has not delivered a Letter of Transmittal.

        The term "Agent's Message" means a message, transmitted by DTC to and received by the Depositary and forming a part of a book-entry confirmation, which states that DTC has

10

received an express acknowledgment from the tendering participant, which acknowledgment states that such participant has received and agrees to be bound by the Letter of Transmittal and that we may enforce the Letter of Transmittal against such participant.

        Only registered holders are authorized to tender their Notes. In all cases, notwithstanding any other provision of the Offer, the payment for the Notes tendered and accepted for payment will be made only after timely receipt by the Depositary of certificates representing tendered Notes or book-entry confirmation, the Letter of Transmittal, or a facsimile thereof, properly completed and duly executed and any required signature guarantees or, in the case of a book-entry transfer, an Agent's Message and other documents required by the Letter of Transmittal.

        If the Notes are held of record in the name of a person other than the signer of the Letter of Transmittal, or if certificates for unpurchased Notes are to be issued to a person other than the registered holder, the Notes must be endorsed or accompanied by appropriate instruments of transfer entitling the signer to tender the Notes on behalf of the registered holder, in any case signed exactly as the name of the registered holder appears on the Notes, with the signatures on the certificates or instruments of transfer guaranteed as described below.

        Need for Signature Guarantee.    Signatures on a Letter of Transmittal must be guaranteed by a recognized participant (each, a "Medallion Signature Guarantor") in the Securities Transfer Agents Medallion Program, unless the tendered Notes are tendered:

•

by the registered holder of such Notes, or by a participant in DTC whose name appears on a security position listing as the owner of such Notes, and that holder has not completed either of the boxes titled "A. Special Issuance/Delivery Instructions" or "B. Special Payment Instructions" on the Letter of Transmittal; or



•

for the account of a firm that is a member of a registered national securities exchange or the National Association of Securities Dealers, Inc. or is a commercial bank or trust company having an office in the United States (each, an "Eligible Institution").

        Book-Entry Delivery of the Notes.    Within two business days after the date of this Statement, the Depositary will establish an account with respect to the Notes at DTC for purposes of the Offer. Any financial institution that is a participant in the DTC system may make book-entry delivery of Notes by causing DTC to transfer such Notes into the Depositary's account in accordance with DTC's procedure for such transfer. Although delivery of Notes may be effected through book-entry at DTC, the Letter of Transmittal or facsimile thereof, with any required signature guarantees, or an Agent's Message in lieu of the Letter of Transmittal, and any other required documents, must be transmitted to and received by the Depositary on or before the Expiration Date at its address set forth on the back cover of this Statement. DELIVERY OF DOCUMENTS TO DTC DOES NOT CONSTITUTE DELIVERY TO THE DEPOSITARY.

        Guaranteed Delivery.    If a holder desires to tender Notes under the Offer and the holder's certificates are not immediately available or cannot be delivered to the Depositary on or before the Expiration Date, or the procedure for book-entry transfer cannot be completed on or before the Expiration Date, or if time will not permit all required documents to reach the Depositary on or before the Expiration Date, the Notes may nevertheless be validly tendered, provided that all of the following conditions are satisfied:

•

the tender is made by or through an Eligible Institution;



•

the Depositary receives by hand, mail, overnight courier, telegram or facsimile transmission, on or before the Expiration Date, a properly completed and duly executed

11

Notice of Guaranteed Delivery substantially in the form provided with this Statement, including (where required) a signature guarantee by an Eligible Institution in the form set forth in such Notice of Guaranteed Delivery; and

•

the certificates for all tendered Notes, in proper form for transfer, or confirmation of a book-entry transfer of Notes into the Depositary's account at DTC, together with a properly completed and duly executed Letter of Transmittal, or a manually signed facsimile thereof, including any required signature guarantees, or an Agent's Message, and any other documents required by the Letter of Transmittal, are received by the Depositary within three New York Stock Exchange trading days after the date of receipt by the Depositary of the Notice of Guaranteed Delivery.

        General.    The tender of Notes pursuant to the Offer by one of the procedures set forth above will constitute:

•

the tendering holder's acceptance of the terms and conditions of the Offer; and



•

a representation and warranty by the tendering holder that:



•

such holder has the full power and authority to tender, sell, assign and transfer the tendered Notes; and



•

when the same are accepted for payment by us, we will acquire good and unencumbered title to such Notes, free and clear of all liens, restrictions, charges and encumbrances and not subject to adverse claims or rights.

        The acceptance for payment by us of Notes will constitute a binding agreement between us and the tendering holder upon the terms and subject to the conditions of the Offer.

        THE METHOD OF DELIVERY OF THE LETTER OF TRANSMITTAL, CERTIFICATES FOR NOTES AND ALL OTHER REQUIRED DOCUMENTS IS AT THE ELECTION AND RISK OF THE TENDERING HOLDER. IF A HOLDER CHOOSES TO DELIVER BY MAIL, THE RECOMMENDED METHOD IS BY REGISTERED MAIL WITH RETURN RECEIPT REQUESTED, PROPERLY INSURED. IN ALL CASES, SUFFICIENT TIME SHOULD BE ALLOWED TO ENSURE TIMELY DELIVERY.

        Form and Validity.    All questions as to the form of all documents and the validity, eligibility, including time of receipt, acceptance for payment and withdrawal of tendered Notes will be determined by us, in our sole discretion, and our determination will be final and binding. We reserve the absolute right to reject any and all tenders of Notes that we determine are not in proper form or the acceptance for payment of or payment for which may, in the opinion of our counsel, be unlawful. We also reserve the absolute right in our sole discretion to waive any defect or irregularity in the tender of Notes of any holder. Our interpretation of the terms and conditions of the Offer, including the instructions in the Letter of Transmittal, will be final and binding. None of us, the Dealer Manager, the Depositary, the Information Agent or any other person will be under any duty to give notification of any defects or irregularities in tenders or any notices of withdrawal or will incur liability for failure to give any such notification.

Withdrawal of Tendered Notes

        When Notes may be Withdrawn.    You may withdraw your tendered Notes at any time on or before the Expiration Date. You may also withdraw your Notes if we have not accepted them for payment by August 5, 2005. A withdrawal of previously tendered Notes may not be rescinded. Any Notes properly withdrawn will be deemed not validly tendered for purposes of the Offer unless such Notes are properly re-tendered.

12

        Holders who have withdrawn their previously tendered Notes may re-tender Notes at any time on or before the Expiration Date, by following one of the procedures described in "—Procedure for Tendering Notes." In the event of a termination of the Offer, the Notes tendered pursuant to the Offer will be promptly returned to the tendering holder.

        Procedure for Withdrawing Notes.    For a withdrawal of Notes to be effective, a written or facsimile transmission notice of withdrawal or a Request Message (as defined below) must be timely received by the Depositary at its address set forth on the back cover of this Statement. The withdrawal notice must:

•

specify the name of the person who tendered the Notes to be withdrawn;



•

contain a description of the Notes to be withdrawn;



•

specify the certificate numbers shown on the particular certificates evidencing such Notes and the aggregate principal amount represented by such Notes (unless such Notes were tendered by book-entry transfer); and



•

be signed by the holder of such Notes in the same manner as the original signature on the Letter of Transmittal, including any required signature guarantees.

        Alternatively, the withdrawal notice must be accompanied by evidence satisfactory to us, in our sole discretion, that the person withdrawing the tender has succeeded to the beneficial ownership of the Notes. In addition, any such notice of withdrawal must specify, in the case of Notes tendered by delivery of certificates for such Notes, the name of the registered holder, if different from that of the tendering holder, or, in the case of Notes tendered by book-entry transfer, the name and number of the account at DTC to be credited with the withdrawn Notes. The signature on the notice of withdrawal must be guaranteed by an Eligible Institution unless such Notes have been tendered for the account of an Eligible Institution. In lieu of submitting a written, telegraphic or facsimile transmission notice of withdrawal, DTC participants may electronically transmit a request for withdrawal to DTC. DTC will then edit the request and send a request message (a "Request Message") to the Depositary. If certificates for the Notes to be withdrawn have been delivered or otherwise identified to the Depositary, a Request Message or a signed notice of withdrawal will be effective immediately upon receipt by the Depositary of a Request Message or a written or facsimile transmission notice of withdrawal even if physical release is not yet effected. Any Notes properly withdrawn will be deemed to be not validly tendered for purposes of the Offer. Withdrawals of Notes can be accomplished only in accordance with the foregoing procedures.

        If a holder tenders its Notes in the Offer, such holder may convert its Notes only if such holder withdraws its Notes prior to the time such holder's right to withdraw has expired. The Notes are convertible into shares of our common stock at a conversion rate (subject to adjustment) of 12.3458 shares per $1,000 principal amount, which is equal to a conversion price of $81.00 per share.

        Form and Validity.    All questions as to the form and validity, including time of receipt, of notices of withdrawal of tenders will be determined by us, in our sole discretion, which determination will be final and binding. None of us, the Dealer Manager, the Depositary, the Information Agent or any other person will be under any duty to give notification of any defects or irregularities in any notices of withdrawal or be subject to any liability for failure to give any such notification.

Conditions to the Offer

        The Offer is not conditioned on a minimum principal amount of Notes being tendered. Notwithstanding any other provision of the Offer, we may terminate or amend the Offer or

13

may postpone the acceptance for payment of, or the purchase of and payment for, Notes tendered, subject to the rules under the Exchange Act, if at any time on or before the Expiration Date any of the following events has occurred (or been determined by us to have occurred) that, in our reasonable judgment and regardless of the circumstances giving rise to the event or events, makes it inadvisable to proceed with the Offer, with acceptance of the Notes for payment or with payment for the Notes:

          (i)  there is pending or has been threatened or instituted any action, proceeding or investigation by or before any court or governmental regulatory or administrative agency or authority or tribunal, domestic or foreign, which (a) challenges the making of the Offer, the acquisition of Notes pursuant to the Offer or otherwise relates in any manner to the Offer or (b) in our reasonable judgment, could have a material adverse effect on the business, financial condition, income, operations or prospects of Cephalon and its subsidiaries, taken as a whole (a "Material Adverse Effect");

         (ii)  there has been any material adverse development, in our reasonable judgment, with respect to any action, proceeding or investigation concerning Cephalon existing on the date hereof;

        (iii)  a statute, rule, regulation, judgment, order, stay or injunction shall have been threatened, proposed, sought, promulgated, enacted, entered, enforced or deemed to be applicable by any court or governmental regulatory or administrative agency, authority or tribunal, domestic or foreign, which, in our reasonable judgment, would or might directly or indirectly prohibit, prevent, restrict or delay consummation of the Offer or that could have a Material Adverse Effect;

        (iv)  there has been or is likely to occur any event or series of events that, in our reasonable judgment, would or might prohibit, prevent, restrict or delay consummation of the Offer or that will, or is reasonably likely to, materially impair the contemplated benefits to Cephalon of the Offer, or otherwise result in the consummation of the Offer not being, or not being reasonably likely to be, in the best interests of Cephalon;

         (v)  there has been (a) any general suspension of, shortening of hours for or limitation on prices for trading in securities in the United States securities or financial markets for a period in excess of 24 hours, (b) a material impairment in the trading market for debt securities, (c) a declaration of a banking moratorium or any suspension of payments in respect of banks by federal or state authorities in the United States (whether or not mandatory), (d) a commencement of a war, armed hostilities, act of terrorism or other national or international crisis, (e) any limitation (whether or not mandatory) by any governmental authority on, or other event having a reasonable likelihood of affecting, the extension of credit by banks or other lending institutions in the United States, (f) any material change in the United States currency exchange rates or a suspension of, or limitations on, the markets therefor (whether or not mandatory) or (g) in the case of any of the foregoing existing at the time of the commencement of the Offer, a material acceleration or worsening thereof; or

        (vi)  there has been or is likely to occur any change or development, including without limitation, a change or development involving a prospective change, in or affecting the business or financial affairs of Cephalon and its subsidiaries which, in our reasonable judgment, could or might prohibit, restrict or delay consummation of the Offer or impair the contemplated benefits of the Offer to Cephalon or might be material in deciding whether to accept any tenders of Notes.

        IMPORTANT: The above conditions are for our sole benefit and may be asserted by us regardless of the circumstances, including any action or inaction by us, giving rise to such

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condition or may be waived by us in whole or in part at any time and from time to time in our sole discretion on or prior to the Expiration Date. The failure by us at any time to exercise any of the foregoing rights will not be deemed a waiver of any other right, and each right will be deemed an ongoing right which may be asserted at any time and from time to time.

Extension, Waiver, Amendment and Termination

        We expressly reserve the right, in our sole discretion at any time or from time to time, subject to applicable law:

•

to extend the Expiration Date and thereby delay acceptance for payment of, and the payment for, any Notes, by giving written notice of such extension to the Depositary and making a public announcement of the extension;



•

to amend the Offer in any respect, by giving written notice of such amendment to the Depositary and making a public announcement of the amendment; or



•

to waive in whole or in part any condition to the Offer and accept for payment and purchase all Notes validly tendered and not validly withdrawn on or before the Expiration Date.

        We expressly reserve the right, in our sole discretion, subject to applicable law, to terminate the Offer. If any of the events set forth under "—Conditions to the Offer" has occurred, we reserve the right, in our sole discretion, to (i) terminate the Offer and reject for payment and not pay for any Notes tendered that we have not already accepted for payment and paid for and (ii) subject to applicable law, postpone payment for any tendered Notes. If we elect to terminate the Offer or postpone payment for tendered Notes, we will give written notice to the Depositary and make a public announcement of such termination or postponement. Our reservation of the right to delay payment for Notes that we have accepted for payment is limited by Rule 13e-4(f)(5), which requires that we pay the consideration offered or return the Notes tendered promptly after the termination or withdrawal of the Offer.

        If we materially change the terms of the Offer or the information concerning the Offer, or if we waive a material condition of the Offer, we will disseminate additional tender offer materials and extend the Offer to the extent required by Rule 13e-4(d)(2) and Rule 13e-4(e)(3) under the Exchange Act.

        We will notify you as promptly as practicable of any other extension, waiver, amendment or termination by public announcement, with the announcement in the case of an extension to be issued no later than 9:00 a.m., New York City time, on the first business day after the previously scheduled Expiration Date. Without limiting the manner in which we may choose to make any public announcement, we will have no obligation to publish, advertise or otherwise communicate any such public announcement other than by issuing a release to the Dow Jones News Service.

        If we terminate the Offer, we will promptly give notice of the termination to the Depositary, and all Notes previously tendered will be returned promptly to the tendering holders thereof. If the Offer is withdrawn or otherwise not completed, the purchase price will not be paid or become payable to holders of Notes who have validly tendered their Notes in the Offer.

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CERTAIN SIGNIFICANT CONSIDERATIONS

        In deciding whether to participate in the Offer, you should consider the following factors, in addition to the other information presented in this Statement and the documents that we incorporate by reference into this Statement, including the important factors described in "Special Note Regarding Forward-Looking Statements." These are not the only risks we face. Any of these risks, as well as other risks and uncertainties that we do not know about now or that we do not think are important, could seriously harm our business and financial results and cause the value of the Notes to decline, which in turn could cause you to lose all or part of your investment.

        Limited Trading Market.    The Notes are eligible for trading on The Portal Market, a subsidiary of The Nasdaq Stock Market, Inc., which permits the offer and sale of the Notes among only "qualified institutional buyers" as defined in Rule 144A of the Securities Act of 1933, as amended. There is no existing public trading market for the Notes. Quotations for securities that are not widely traded, such as the Notes, may differ from actual trading prices and should be viewed as approximations. Holders are urged to contact their brokers with respect to current information regarding the Notes. To the extent that some but not all of the Notes are purchased pursuant to the Offer, any existing trading market for the remaining Notes may become more limited. A debt security with a smaller outstanding principal amount available for trading (the "float") may command a lower price than would a comparable debt security with a greater float. The reduced float may also make the trading price of Notes that are not accepted for payment pursuant to the Offer more volatile. Consequently, the liquidity, market value and price volatility of Notes that remain outstanding may be adversely affected. Holders of Notes not purchased in the Offer may attempt to obtain quotations for such Notes from their brokers; however, there can be no assurance that any trading market will exist for such Notes following consummation of the Offer. The extent of the public market for the Notes following consummation of the Offer depends upon the number of holders remaining at such time, the interest in maintaining a market in such Notes on the part of securities firms and other factors.

        Existence of Debt.    We have now and, after consummating the Offer will continue to have, a significant amount of indebtedness. As of March 31, 2005, we had $1,288.3 million of indebtedness outstanding, of which $19.3 million consisted of senior indebtedness and $1,269.0 million consisted of subordinated indebtedness, including the Notes. In addition, on June 7, 2005, we issued and sold $800 million in aggregate principal amount of our 2.00% Convertible Senior Subordinated Notes due June 1, 2015. There are no restrictions on our use of our existing cash, cash equivalents and investments, and we cannot be sure that these funds will be available or sufficient in the future to enable us to repay or refinance our indebtedness. In the future, these factors, among other things, could make it difficult for us to service, repay or refinance our indebtedness or obtain additional financing in the future; limit our flexibility in planning for, or reacting to changes in, our business; and make us more vulnerable in the event of a downturn in our business. Unless we are able to generate cash flow from operations that, together with our available cash on hand, is sufficient to repay our indebtedness, we will be required to raise additional funds. Because the financing markets may be unwilling to provide funding to us or may only be willing to provide funding on terms that we would consider unacceptable, we may not have cash available or be able to obtain funding to permit us to meet our repayment obligations, thus adversely affecting the market price for our securities.

        Ranking of Notes.    The Notes that remain outstanding upon consummation of the Offer will remain our obligations with regard to payments of principal, interest and premium.

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However, the Notes are unsecured and subordinated to our existing and future senior indebtedness and senior subordinated indebtedness. In the event of our insolvency, liquidation, reorganization or payment default on senior indebtedness, we will not be able to make payments on Notes that remain outstanding upon consummation of the Offer until we have paid in full all of our senior indebtedness and senior subordinated indebtedness. We may, therefore, not have sufficient assets to pay the amounts due on such Notes. We are not prohibited from incurring debt under the Indenture, including debt senior to, on parity with or subordinate to the Notes. If we incur additional debt, our ability to pay amounts due on Notes that remain outstanding upon consummation of the Offer could be adversely affected.

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

        In addition to historical facts or statements of current condition, this Statement and the documents that are incorporated by reference into this Statement may contain forward-looking statements. Forward-looking statements contained in this Statement constitute our expectations or forecasts of future events as of the date of this Statement and are not statements of historical fact. You can identify these statements by the fact that they do not relate strictly to historical or current facts. These statements may include words such as "anticipate," "will," "estimate," "expect," "project," "intend," "should," "plan," "believe," "hope" and other words and terms of similar meaning in connection with any discussion of, among other things, future operating or financial performance, strategic initiatives and business strategies, regulatory or competitive environments, our intellectual property and product development. In particular, these forward-looking statements include, among others, statements about:

•

our dependence on sales of PROVIGIL tablets, ACTIQ and GABITRIL in the United States and the market prospects and future marketing efforts for these products;



•

any potential expansion of the authorized uses of our existing products or success with respect to potential product candidates;



•

our anticipated scientific progress in our research programs and our development of potential pharmaceutical products including our ongoing or planned clinical trials, the timing and costs of these trials and the likelihood or timing of revenues from these products, if any;



•

the timing and unpredictability of regulatory approvals;



•

our ability to adequately protect our technology and enforce our intellectual property rights and the future expiration of patent and/or regulatory exclusivity on certain of our products;



•

our future cash flow, our ability to service or repay our existing debt and our ability to raise additional funds, if needed, in light of our current and projected level of operations;



•

the ongoing investigations by and discussions with the U.S. Attorney's Office in Philadephia, the Attorney General in Pennsylvania and the Office of Attorney General in Connecticut; and



•

other statements regarding matters that are not historical facts or statements of current condition.

        Any or all of our forward-looking statements in this Statement and in the documents we have referred you to may turn out to be wrong. They can be affected by inaccurate assumptions we might make or by known or unknown risks and uncertainties. Therefore, you

17

should not place undue reliance on any such forward-looking statements. The factors that could cause actual results to differ from those expressed or implied by our forward-looking statements include, among others:

•

the acceptance of our products by physicians and patients in our current markets and new markets;



•

our ability to obtain regulatory approvals of new products or for new formulations or indications for our existing products;



•

scientific or regulatory setbacks with respect to research programs, clinical trials, manufacturing activities and/or our existing products;



•

unanticipated cash requirements to support current operations, expand our business or incur capital expenditures;



•

the inability to adequately protect our key intellectual property rights, including as a result of an adverse adjudication with respect to the PROVIGIL or ACTIQ patent litigation;



•

the loss of key management or scientific personnel;



•

the activities of our competitors in the industry, including the filing of Abbreviated New Drug Applications with a Paragraph IV certification for GABITRIL or any product containing modafinil or the entry of a generic competitor to ACTIQ;



•

market conditions in the biotechnology industry that make raising capital or consummating acquisitions difficult, expensive or both;



•

enactment of new government, regulations, court decisions, regulatory interpretations or other initiatives that are adverse to us or our interests;



•

an adverse outcome of pending or future investigations by government authorities with respect to our activities; and



•

failure to realize expected or anticipated benefits from past or future acquisitions, whether as a result of unidentified risks, integration difficulties, regulatory setbacks or otherwise.

        We do not intend to update publicly any forward-looking statement, whether as a result of new information, future events or otherwise, except as may be required by Rule 13e-4(d)(2) under the Exchange Act or any other applicable law.

WHERE YOU CAN FIND MORE INFORMATION

        We file periodic reports, proxy and information statements and other information with the SEC. Copies of these reports, proxy and information statements and other information may be examined without charge at the Public Reference Room of the SEC, 100 F Street, N.E., Washington, D.C. 20549, or on the Internet at www.sec.gov. Copies of all or a portion of these materials can be obtained from the Public Reference Room of the SEC upon payment of prescribed fees. Please call the SEC at 1-800-SEC-0330 for further information about the Public Reference Room.

DOCUMENTS INCORPORATED BY REFERENCE

        We are "incorporating by reference" into this Statement certain information that we file with the SEC. This means that we can disclose important business, financial and other information to you by referring you to the documents containing this information. We

18

incorporate by reference into this Statement the documents listed below (which filed documents do not include any portion thereof containing information furnished under either Item 2.02 or Item 7.01, or any related exhibit, of any Current Report on Form 8-K):

•

Annual Report on Form 10-K for the year ended December 31, 2004, filed with the SEC on March 15, 2005;



•

Quarterly Report on Form 10-Q for the quarter ended March 31, 2005, filed with the SEC on May 9, 2005; and



•

Current Reports on Form 8-K and Form 8-K/A filed with the SEC on January 6, January 31, February 3, February 8, February 11, February 17, April 26, May 9, May 13, May 23, June 7 and June 8, 2005.

        We will provide without charge to each person to whom a copy of this Statement is delivered, including any beneficial owner, upon the written or oral request of such person, a copy of any or all of the documents incorporated by reference (other than exhibits to such documents, unless such exhibits are specifically incorporated by reference into the information that this Statement incorporates). Requests should be directed to:

Cephalon, Inc.
41 Moores Road
Frazer, PA 19355
Attn: John E. Osborn
Senior Vice President,
General Counsel and Secretary
Telephone: (610) 344-0200

CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES

        The following is a summary of certain of the material U.S. federal income tax consequences related to a sale of Notes by beneficial owners of Notes (solely as used in this section, "Holders") pursuant to the Offer. Unless otherwise stated, this summary deals only with Holders who hold the Notes as capital assets.

        As used herein, "U.S. Holders" are any Holders of Notes that are, for U.S. federal income tax purposes, (i) citizens or residents of the United States, (ii) corporations created or organized in, or under the laws of, the United States, any state thereof or the District of Columbia, (iii) estates, the income of which is subject to United States federal income taxation regardless of its source, or (iv) trusts that are treated as United States persons under section 7701(a)(30) of the Internal Revenue Code of 1986, as amended (the "Code"). As used herein, "Non-U.S. Holders" are Holders, other than partnerships, that are not U.S. Holders as defined above. If a partnership (including for this purpose any entity treated as a partnership for U.S. federal income tax purposes) is a Holder, the treatment of a partner in the partnership will generally depend upon the status of the partner and upon the activities of the partnership. Partnerships and partners in such partnerships should consult their tax advisors about the U.S. federal income tax consequences of a sale of Notes pursuant to the Offer.

        This summary does not describe all of the U.S. federal income tax consequences that may be relevant to a Holder in light of its particular circumstances. For example, it does not deal with special classes of Holders such as banks, thrifts, real estate investment trusts, regulated investment companies, insurance companies, dealers or traders in securities or currencies, or tax-exempt investors. It also does not discuss Notes held as part of a hedge, straddle, "synthetic security," or other integrated transaction. This summary does not address the U.S. federal income tax consequences to (i) persons that have a functional currency other

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than the U.S. dollar, (ii) certain United States expatriates or (iii) shareholders, partners or beneficiaries of a Holder. Further, it does not include any description of any alternative minimum tax consequences, estate or gift tax consequences or the tax laws of any state or local government or of any foreign government that may be applicable to the Notes.

        This summary is based on the Code, the Treasury regulations promulgated thereunder and administrative and judicial interpretations thereof, all as of the date hereof, and all of which are subject to change or differing interpretations, possibly on a retroactive basis.

Holders should consult their own tax advisors regarding the federal, state, local and foreign income, franchise, personal property and any other tax consequences of tendering the Notes pursuant to the Offer.

U.S. Holders

Gain or Loss Upon Sale

        The sale of a Note by a U.S. Holder pursuant to the Offer will be a taxable transaction for U.S. federal income tax purposes. Subject to the market discount rules discussed below, a U.S. Holder will generally recognize capital gain or loss in an amount equal to the difference between the amount realized on the sale of a Note and the U.S. Holder's adjusted tax basis in the Note sold. The amount realized on the sale of a Note pursuant to the Offer will be equal to the amount of cash received in exchange for the Note (other than amounts attributable to accrued but unpaid interest, which will be treated as a payment of interest and will be ordinary income to the U.S. Holder unless such U.S. Holder has previously included such amounts in income under its regular method of accounting, or otherwise has a tax basis in such interest).

        Under the market discount rules, a portion of any gain realized on the sale of a Note by a U.S. Holder who acquired the Note with "market discount" may be characterized as ordinary income rather than capital gain. Market discount is equal to the excess of the face amount of a Note over the U.S. Holder's tax basis in such Note immediately after its acquisition by such U.S. Holder. Unless the U.S. Holder has elected to include market discount in income currently as it accrues or the market discount comes within a statutory de minimis exception, any gain realized by a U.S. Holder on the sale of a Note having market discount will be treated as ordinary income to the extent of the market discount that has accrued while such Note was held by the U.S. Holder.

        Generally, a U.S. Holder's adjusted tax basis in a Note will be equal to the cost of the Note, increased, if applicable, by any market discount previously included in income by such U.S. Holder pursuant to an election to include market discount in income currently as it accrues, and reduced by the accrual of any amortizable bond premium which such U.S. Holder has previously elected to deduct from gross income.

        In the case of a U.S. Holder other than a corporation, preferential tax rates may apply to capital gain recognized on the sale of a Note if the U.S. Holder's holding period for the Note exceeds one year. Subject to certain limited exceptions, a capital loss recognized on the sale of a Note cannot be applied to offset ordinary income for U.S. federal income tax purposes.

Information Reporting and Backup Withholding

        In general, information reporting requirements will apply to the payment of the gross proceeds (including any amounts attributable to accrued but unpaid interest) of the Offer to a U.S. Holder and a backup withholding tax will apply to such payments if the U.S. Holder fails to comply with certain certification procedures or establish an exemption from backup

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withholding. The backup withholding tax rate is currently 28%. Any amounts withheld under the backup withholding rules from a payment to a U.S. Holder will be allowed as a credit against such Holder's U.S. federal income tax liability and may entitle the Holder to a refund, provided that the required information is furnished to the Internal Revenue Service.

Non-U.S. Holders

Gain or Loss Upon Sale

        Subject to the discussion below concerning information reporting and backup withholding, a Non-U.S. Holder generally will be subject to U.S. federal income tax on any gain realized on the sale of a Note pursuant to the Offer if (1) the gain is effectively connected with the conduct of a trade or business within the United States by such Non-U.S. Holder, (2) in the case of a Non-U.S. Holder who is an individual, such Holder is present in the United States for a period or periods aggregating 183 days or more during the taxable year of the disposition, and either (A) such holder has a "tax home" in the United States or (B) the disposition is attributable to an office or other fixed place of business maintained by such Holder in the United States or (3) Cephalon is or has been treated as a United States real property holding corporation for U.S. federal income tax purposes. We believe that we are not and have not been treated as a United States real property holding corporation for these purposes, and therefore Non-U.S. Holders should not be subject to U.S. federal income tax under clause (3) above.

        If an individual Non-U.S. Holder falls under clause (1) above, such individual generally will be taxed on the net gain derived from the sale in the same manner as a U.S. Holder. See "—U.S. Holders—Gain or Loss Upon Sale" above. If an individual Non-U.S. Holder falls under clause (2) above, such individual generally will be subject to a 30% tax on the gain derived from the sale, which may be offset by certain United States capital losses. Individual Non-U.S. Holders who have spent (or expect to spend) 183 days or more in the United States in the taxable year of the sale are urged to consult their tax advisors as to the U.S. federal income tax consequences of such sale. If a Non-U.S. Holder that is a foreign corporation falls under clause (1), it generally will be taxed on the net gain derived from the sale in the same manner as a U.S. Holder as described in "—U.S. Holders—Gain or Loss Upon Sale" above and, in addition, may be subject to an additional "branch profits" tax on such effectively connected income at a 30% rate (or such lower rate as may be specified by an applicable income tax treaty). Non-U.S. Holders should consult their own tax advisors concerning the U.S. federal income tax consequences of a sale of the Notes pursuant to the Offer.

Accrued Unpaid Interest

        Subject to the discussion below concerning information reporting and backup withholding, amounts received by a tendering Non-U.S. Holder attributable to accrued but unpaid interest on the Notes will not be subject to U.S. federal income or withholding tax if such amounts are not effectively connected with the conduct of a trade or business within the United States by such Non-U.S. Holder and the Non-U.S. Holder (1) does not actually or constructively own 10% of more of the total combined voting power of all classes of Cephalon's stock entitled to vote; (2) is not a controlled foreign corporation that is related to Cephalon through stock ownership; (3) is not a bank which acquired the Notes in consideration for an extension of credit made pursuant to a loan agreement entered into in the ordinary course of business; and (4) either (A) certifies to the payor or the payor's agent, under penalties of perjury, that it is not a United States person and provides its name, address, and certain other information on a properly executed Form W-8BEN or a suitable substitute form or (B) a securities clearing organization, bank or other financial institution that

21

holds customer securities in the ordinary course of its trade or business and holds the Notes in such capacity, certifies to the payor or the payor's agent, under penalties of perjury, that such a statement has been received from the beneficial owner by it or by a financial institution between it and the beneficial owner, and furnishes the payor or the payor's agent with a copy thereof. The applicable Treasury regulations also provide alternative methods for satisfying the certification requirements of clause (4), above. If a Non-U.S. Holder holds the Notes through certain foreign intermediaries or partnerships, such Non-U.S. Holder and the foreign intermediary or partnership may be required to satisfy certification requirements under applicable Treasury regulations.

        Except to the extent that an applicable income tax treaty otherwise provides, a Non-U.S. Holder will be taxed with respect to amounts attributable to interest paid on the Notes in the same manner as a U.S. Holder if such amounts are effectively connected with a United States trade or business of the Non-U.S. Holder. Effectively connected income received or accrued by a corporate Non-U.S. Holder may also, under certain circumstances, be subject to an additional "branch profits" tax at a 30% rate (or, if applicable, at a lower tax rate specified by an applicable income tax treaty). Although effectively connected income is subject to income tax, and may be subject to the branch profits tax, it is not subject to withholding tax if the Non-U.S. Holder delivers a properly executed Form W-8ECI (or successor form) to the payor or the payor's agent.

Information Reporting and Backup Withholding

        United States backup tax withholding will not apply to interest payments to a Non-U.S. Holder if the requirements described in clause (4) in the first paragraph of "—Accrued Unpaid Interest" above are satisfied with respect to the Non-U.S. Holder unless the payor has actual knowledge or reason to know that the Non-U.S. Holder is a United States person. Information reporting requirements may apply with respect to interest payments on the Notes, in which event the amount of interest paid and tax withheld (if any) with respect to each Non-U.S. Holder will be reported annually to the Internal Revenue Service. Information reporting requirements and backup tax withholding will not apply to any payment of the proceeds of the sale of Notes effected outside the United States by a foreign office of a "broker" as defined in applicable Treasury regulations, unless such broker (1) is a United States person as defined in the Code, (2) is a foreign person that derives 50% or more of its gross income for certain periods from the conduct of a trade or business in the United States, (3) is a controlled foreign corporation for U.S. federal income tax purposes or (4) is a foreign partnership with certain United States connections. Payment of the proceeds of any such sale effected outside the United States to a Non-U.S. Holder by a foreign office of any broker that is described in the preceding sentence may be subject to information reporting requirements but not backup withholding tax (unless the payor has actual knowledge that the payee is a United States person). Payment of the proceeds of any such sale to or through the United States office of a broker is subject to information reporting and backup withholding requirements unless the beneficial owner satisfies certain certification requirements or otherwise establishes an exemption.

The U.S. federal income tax discussion set forth above is included for general information only and may not be applicable depending upon a Holder's particular situation. Holders should consult their tax advisors with respect to the tax consequences to them of the sale of Notes pursuant to the Offer, including the tax consequences under state, local, foreign and other tax laws and the possible effects of changes in U.S. federal or other tax laws.

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THE DEALER MANAGER, DEPOSITARY AND INFORMATION AGENT

Dealer Manager

        Deutsche Bank Securities Inc. is acting as the Dealer Manager for the Offer. Pursuant to the Dealer Manager Agreement, we will reimburse Deutsche Bank Securities Inc. for its reasonable out-of-pocket expenses. We have agreed to indemnify Deutsche Bank Securities Inc. and its affiliates against certain liabilities in connection with the Offer, including liabilities under the U.S. federal securities laws.

        Deutsche Bank Securities Inc. and its affiliates have engaged in, and may in the future engage in, investment banking, commercial banking, financial advisory services and other commercial dealings in the ordinary course of business with us. They have received, and expect to receive, customary fees and commissions for these transactions. At any time, Deutsche Bank Securities Inc. may trade the Notes or our common stock for its own account or for the accounts of customers and, accordingly, may hold a long or short position in the Notes or our common stock.

        Any holder who has questions concerning the terms of the Offer may contact the Dealer Manager at the address set forth on the back cover page of this Statement.

Depositary and Information Agent

        The Depositary for the Offer is U.S. Bank National Association. All deliveries, correspondence and questions sent or presented to the Depositary relating to the Offer should be directed to the address or telephone number set forth on the back cover of this Statement. As compensation for its services, the Depositary will receive a flat fee in a customary amount, will be reimbursed for reasonable out-of-pocket expenses and will be indemnified against liabilities in connection with its services, including liabilities under the U.S. federal securities laws.

        Morrow & Co., Inc. is acting as the Information Agent for the Offer. We will pay the Information Agent reasonable and customary compensation for such services, plus reimbursement for out-of-pocket expenses. All inquiries and correspondence addressed to the Information Agent relating to the Offer should be directed to the address or telephone numbers set forth on the back cover page of this Statement.

        Brokers, dealers, commercial banks and trust companies will be reimbursed by us for customary mailing and handling expenses incurred by them in forwarding material to their customers. We will not pay any fees or commissions to any broker, dealer or other person, other than the Dealer Manager, in connection with the solicitation of tenders of Notes pursuant to the Offer.

23

MISCELLANEOUS

        Pursuant to Rule 13e-4 under the Exchange Act, we have filed with the SEC a tender offer statement on Schedule TO that contains additional information with respect to the Offer. The Schedule TO, including the exhibits and any amendments to the Schedule TO, may be examined, and copies may be obtained, at the same places and in the same manner as described in the sections titled "Documents Incorporated by Reference" and "Where You Can Find More Information."

        The Offer is being made to all holders of Notes. We are not aware of any jurisdiction in which the making of the Offer is prohibited by administrative or judicial action pursuant to a state statute. If we become aware of any jurisdiction where the making of the Offer is so prohibited, we will make a good faith effort to comply with any such statute. If, after such good faith effort, we cannot comply with any applicable statute, the Offer will not be made to (nor will tenders be accepted from or on behalf of) the holders in such jurisdiction.

        The statements contained herein are made as of the date hereof, and the delivery of this Statement and the accompanying materials will not, under any circumstances, create any implication that the information contained herein is correct at any time subsequent to the date hereof.

24

THE DEPOSITARY FOR THE OFFER IS:

U.S. BANK NATIONAL ASSOCIATION

By Mail, Overnight Mail, Courier or Hand:
U.S. Bank National Association
West Side Flats Operations Center
60 Livingston Avenue
St. Paul, MN 55107
Attn: Specialized Finance
For Information: (800) 934-6802

By Facsimile:
(651) 495-8158
Attn: Specialized Finance
Confirm by Receipt of Facsimile Only: (651) 495-3511

Any questions regarding the terms of the Offer may be directed to the Dealer Manager at the address below.

THE DEALER MANAGER FOR THE OFFER IS:

DEUTSCHE BANK SECURITIES

60 Wall Street
New York, New York
10005

Any questions or requests for assistance or additional copies of this Statement, the Letter of Transmittal or other materials may be directed to the Information Agent at the telephone number and address below. You may also contact your broker, dealer, commercial bank or trust company or nominee for assistance concerning the Offer.

THE INFORMATION AGENT FOR THE OFFER IS:

445 Park Avenue, 5th Floor
New York, New York 10022
Email: ceph.info@morrowco.com
Banks and Brokerage Firms, Please Call (800) 654-2468
Noteholders, Please Call (800) 607-0088
All Others Call Collect (212) 754-8000

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TABLE OF CONTENTS
IMPORTANT
SUMMARY
CEPHALON
PURPOSE OF THE OFFER
SOURCE AND AMOUNT OF FUNDS
PRICE RANGE OF COMMON STOCK
DIVIDEND POLICY
SUMMARY FINANCIAL INFORMATION
TERMS OF THE OFFER
CERTAIN SIGNIFICANT CONSIDERATIONS
SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS
WHERE YOU CAN FIND MORE INFORMATION
DOCUMENTS INCORPORATED BY REFERENCE
CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES
THE DEALER MANAGER, DEPOSITARY AND INFORMATION AGENT
MISCELLANEOUS

EX-99.A1II

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Exhibit 99(a)(1)(ii)

LETTER OF TRANSMITTAL

CEPHALON, INC.

Offer to Purchase for Cash
any and all of the Outstanding
2¹/2% Convertible Subordinated Notes Due December 2006
of
Cephalon, Inc.

(CUSIP Nos. 156708AD1 and 156708AE9)

Pursuant to the Offer to Purchase
dated June 10, 2005

THE OFFER AND YOUR WITHDRAWAL RIGHTS WILL EXPIRE AT 5 P.M., NEW YORK CITY TIME, ON JULY 11, 2005, UNLESS EXTENDED (SUCH DATE AND TIME, AS IT MAY BE EXTENDED, THE "EXPIRATION DATE").

The Depositary for the Offer is:
U.S. Bank National Association

By Mail, Overnight Mail, Courier or Hand:
U.S. Bank National Association
West Side Flats Operations Center
60 Livingston Avenue
St. Paul, MN 55107
Attn: Specialized Finance
For Information: (800) 934-6802

By Facsimile:
(651) 495-8158
Attn: Specialized Finance
Confirm by Receipt of Facsimile Only: (651) 495-3511

        DELIVERY OF THIS LETTER OF TRANSMITTAL (THIS "LETTER OF TRANSMITTAL") TO AN ADDRESS, OR TRANSMISSION VIA FACSIMILE TO A NUMBER, OTHER THAN AS SET FORTH ABOVE, WILL NOT CONSTITUTE A VALID DELIVERY.

        Capitalized terms used but not defined herein shall have the same meanings given them in the related Offer to Purchase dated June 10, 2005.

        THE INSTRUCTIONS CONTAINED HEREIN SHOULD BE READ CAREFULLY BEFORE THIS LETTER OF TRANSMITTAL IS COMPLETED AND SIGNED.

        This Letter of Transmittal is to be used by the holders of the 2¹/2% Convertible Subordinated Notes due December 2006 (the "Notes") of Cephalon, Inc., a Delaware corporation ("Cephalon"), if:

•

certificates for tendered Notes are to be physically delivered to the Depositary; or



•

tender of Notes is to be made by book-entry transfer to the Depositary's account at The Depository Trust Company ("DTC") pursuant to the procedures for book-entry transfer set forth under the caption "Terms of the Offer—Procedure for Tendering Notes" in the Offer to Purchase;

and, in each case, instructions are not being transmitted through the DTC's Automated Tender Offer Program ("ATOP").

        Holders who are tendering by book-entry transfer to the Depositary's account at DTC can execute the tender through ATOP. DTC participants that are accepting the Offer may transmit their acceptance to DTC, which will verify the acceptance and execute a book-entry delivery to the Depositary's account at DTC. DTC will then transmit an Agent's Message to the Depositary for its acceptance. Delivery of the Agent's Message by DTC will satisfy the terms of the Offer as to execution and delivery of a Letter of Transmittal by the

participant identified in the Agent's Message. DELIVERY OF DOCUMENTS TO DTC DOES NOT CONSTITUTE DELIVERY TO THE DEPOSITARY.

        Only registered holders are authorized to tender their Notes. The undersigned should complete, execute and deliver this Letter of Transmittal to indicate the action the undersigned desires to take with respect to the Offer.

TENDERING OF NOTES

o

CHECK HERE IF CERTIFICATES REPRESENTING TENDERED NOTES ARE BEING DELIVERED HEREWITH.

o

CHECK HERE IF TENDERED NOTES ARE BEING DELIVERED BY BOOK-ENTRY TRANSFER MADE TO THE ACCOUNT MAINTAINED BY THE DEPOSITARY WITH A BOOK-ENTRY TRANSFER FACILITY AND COMPLETE THE FOLLOWING:

Name of Tendering Institution:

Account Number:

Transaction Code Number:

o

CHECK HERE IF CERTIFICATES REPRESENTING TENDERED NOTES ARE BEING DELIVERED PURSUANT TO A NOTICE OF GUARANTEED DELIVERY PREVIOUSLY SENT TO THE DEPOSITARY AND COMPLETE THE FOLLOWING:

Name of Registered Holder(s):

Date of Execution of Notice of Guaranteed Delivery:

Name of Institution with Guaranteed Delivery:

Account Number:

2

        List below the Notes to which this Letter of Transmittal relates. If the space provided is inadequate, list the certificate numbers and principal amounts on a separately executed schedule and affix the schedule to this Letter of Transmittal. Tenders of Notes will be accepted only in principal amounts equal to $1,000 or integral multiples.

DESCRIPTION OF NOTES

		Principal Amount Represented	Principal Amount Tendered**
	Certificate Number(s)*
Name(s) and Address(es) of Holder(s)
	Total Principal Amount of Notes:

*

Need not be completed by holders tendering by book-entry transfer.

**

Unless otherwise specified, it will be assumed that the entire aggregate principal amount of the Notes described above is being tendered. See Instruction 4. Only registered holders may validly tender their Notes pursuant to the Offer.

        The names and addresses of the holders should be printed exactly as they appear on the certificates representing the tendered Notes. The Notes and the principal amount of Notes that the undersigned wishes to tender should be indicated in the appropriate boxes.

3

NOTE: SIGNATURES MUST BE PROVIDED BELOW
PLEASE READ THE ACCOMPANYING INSTRUCTIONS CAREFULLY

Ladies and Gentlemen:

        The undersigned hereby tenders to Cephalon, Inc. ("Cephalon"), upon the terms and subject to the conditions set forth in the Offer to Purchase dated June 10, 2005 (as it may be amended or supplemented from time to time, the "Statement"), receipt of which is acknowledged, and in accordance with this Letter of Transmittal (which together with the Statement, constitutes the "Offer"), the principal amount of Notes indicated in the table above labeled "Description of Notes" under the column heading "Principal Amount Tendered." If nothing is indicated in that column, then it will be assumed that the entire aggregate principal amount represented by the Notes described in the table is being tendered.

        Subject to, and effective upon, the acceptance for payment of the Notes tendered in accordance with the Offer, the undersigned hereby sells, assigns and transfers to, or upon the order of, Cephalon, all right, title and interest in and to all of the tendered Notes. The undersigned hereby irrevocably constitutes and appoints the Depositary the true and lawful agent and attorney-in-fact of the undersigned, with full knowledge that the Depositary also acts as the agent of Cephalon, with respect to tendered Notes, and with full powers of substitution and resubstitution:

•

to present such Notes and all evidences of transfer and authenticity to, or transfer of ownership of, such Notes on the account books maintained by The Depository Trust Company ("DTC") to, or upon the order of, Cephalon; and



•

to receive all benefits and otherwise exercise all rights of beneficial ownership of such Notes,

all in accordance with the terms and conditions of the Offer as described in the Statement. The above granted power of attorney is deemed to be an irrevocable power of attorney coupled with an interest.

        If the undersigned is not the registered holder listed in the box above labeled "Description of Notes" under the column heading "Principal Amount Tendered" or the holder's legal representative or attorney-in-fact, then in order to validly tender, the undersigned must obtain and deliver with this Letter of Transmittal Notes that are endorsed or accompanied by appropriate instruments of transfer entitling the undersigned to tender the Notes on behalf of such registered holder, in any case signed exactly as the name of the registered holder appears on the Notes, with the signatures on the certificates or instruments of transfer guaranteed as described below.

        The undersigned understands that tenders of Notes pursuant to any of the procedures described in the Statement and in the instructions in this Letter of Transmittal will constitute the undersigned's acceptance of the terms and conditions of the Offer. Cephalon's acceptance of such Notes for payment will constitute a binding agreement between the undersigned and Cephalon upon the terms and subject to the conditions of the Offer. For purposes of the Offer, the undersigned understands that Cephalon will be deemed to have accepted for payment (and thereby purchased) tendered Notes, or defectively tendered Notes with respect to which Cephalon has, or has caused to be, waived such defect, if, as and when Cephalon gives written notice to the Depositary of its acceptance for payment of such Notes.

        The undersigned hereby represents and warrants that the undersigned has full power and authority to tender, sell, assign and transfer the Notes tendered hereby, and that when such tendered Notes are accepted for payment by Cephalon, Cephalon will acquire good and unencumbered title to the Notes, free and clear of all liens, restrictions, charges and encumbrances and not subject to adverse claims or rights.

        The undersigned will, upon request, execute and deliver any additional documents deemed by the Depositary or by Cephalon to be necessary or desirable to complete the sale, assignment and transfer of the Notes tendered hereby.

        The undersigned understands that Cephalon reserves the right to transfer or assign, from time to time, in whole or in part, to one or more of its affiliates the right to purchase all or any of the Notes validly tendered pursuant to the Offer. If such assignment occurs, the assignee-affiliate will purchase the Notes validly tendered. However, any such transfer or assignment will not relieve Cephalon of its obligations under the Offer and will not prejudice the undersigned's right to receive the purchase price in exchange for the Notes validly tendered and accepted for payment on the acceptance date.

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        All authority herein conferred or agreed to be conferred by this Letter of Transmittal will not be affected by, and will survive, the death or incapacity of the undersigned, and any obligation of the undersigned hereunder will be binding upon the heirs, executors, administrators, trustees in bankruptcy, personal and legal representatives, successors and assigns of the undersigned.

        The undersigned understands that the delivery and surrender of any Notes is not effective, and the risk of loss of the Notes does not pass to the Depositary, until receipt by the Depositary of this Letter of Transmittal, or a facsimile hereof, properly completed and duly executed, or a properly transmitted Agent's Message, together with all accompanying evidences of authority and any other required documents in form satisfactory to Cephalon. All questions as to the form of all documents and the validity, including time of receipt, acceptance for payment and withdrawal of tendered Notes, will be determined by Cephalon, in its sole discretion, which determination will be final and binding.

        Unless otherwise indicated herein in the box below labeled "A. Special Issuance/Delivery Instructions," the undersigned hereby request(s) that any Notes representing principal amounts not tendered or not accepted for purchase be issued in the name(s) of, and delivered to, the undersigned, and in the case of Notes tendered by book-entry transfer, by credit to the account of DTC. Unless otherwise indicated herein in the box below labeled "B. Special Payment Instructions," the undersigned hereby request(s) that any checks for payments to be made in connection with the Offer be issued to the order of, and delivered to, the undersigned.

        If the "A. Special Issuance/Delivery Instructions" box is completed, the undersigned hereby request(s) that any Notes representing principal amounts not tendered or not accepted for purchase be issued in the name(s) of, and be delivered to, the person(s) at the address(es) therein indicated or credited to the DTC account indicated.

        The undersigned recognizes that Cephalon has no obligation pursuant to the "A. Special Issuance/Delivery Instructions" box to transfer any Notes from the names of the registered holder(s) thereof if it does not accept for purchase any of such tendered Notes. In the event that the "B. Special Payment Instructions" box is completed, the undersigned hereby request(s) that checks for payments to be made in connection with the Offer be issued in the name(s) of, and be delivered to, the person(s) at the address(es) therein indicated.

5

A. SPECIAL ISSUANCE/DELIVERY INSTRUCTIONS
(SEE INSTRUCTIONS 1, 5, 6 AND 7)

        To be completed ONLY if Notes in a principal amount not tendered or not accepted for purchase are to be issued in the name of someone other than the person(s) whose signature(s) appear(s) within this Letter of Transmittal or sent to an address different from that shown in the box entitled "Description of Notes" within this Letter of Transmittal.

Name:
	(Please Print)

Address:
	(Including ZIP Code)
(Tax Identification or Social Security Number) (See Substitute Form W-9 Herein)

Credit unpurchased Notes by book-entry to the DTC account set forth below:

Number of Account Party:

B. SPECIAL PAYMENT INSTRUCTIONS
(SEE INSTRUCTIONS 1, 5, 6 AND 7)

        To be completed ONLY if checks are to be issued in the name of someone other than the person(s) whose signature(s) appear(s) within this Letter of Transmittal or sent to an address different from that shown in the box entitled "Description of Notes" within this Letter of Transmittal.

Name:
	(Please Print)

Address:
	(Including ZIP Code)
(Tax Identification or Social Security Number) (See Substitute Form W-9 Herein)

6

PLEASE SIGN HERE

(TO BE COMPLETED BY ALL TENDERING HOLDERS REGARDLESS OF WHETHER
NOTES ARE BEING PHYSICALLY DELIVERED HEREWITH)

        By completing, executing and delivering this Letter of Transmittal, the undersigned hereby tenders the principal amount of the Notes listed in the box above labeled "Description of Notes" under the column heading "Principal Amount Tendered" (or, if nothing is indicated therein, with respect to the entire aggregate principal amount represented by the Notes described in such box).

        This Letter of Transmittal must be signed by the holder(s) exactly as its name(s) appear(s) on certificate(s) representing Notes, or if tendered by a participant in DTC, exactly as such participant's name appears on a security position listing as the owner of Notes. If signature is by trustees, executors, administrators, guardians, attorneys-in-fact, officers of corporations or others acting in a fiduciary or representative capacity, please set forth full title and see Instruction 5.

Signature(s) of Registered Holder(s) or Authorized Signatory
(See Guarantee Requirement Below)

Dated:

Name(s):

(Please Print)

Capacity:

Address:

(Including ZIP Code)

Area Code and Telephone Number:

Tax Identification or Social Security No.:
	(Complete Accompanying Substitute Form W-9)

MEDALLION SIGNATURE GUARANTEE
(If Required—See Instructions 1 and 5)

Authorized Signature:

Name of Firm:

7

INSTRUCTIONS
FORMING PART OF THE TERMS AND CONDITIONS OF THE OFFER

        1.    Signature Guarantees.    Signatures on this Letter of Transmittal must be guaranteed by a Medallion Signature Guarantor, unless the Notes are tendered by the registered holder of such Notes, or by a participant in DTC whose name appears on a security position listing as the owner of such Notes, and that holder has not completed either of the boxes entitled "A. Special Issuance/Delivery Instructions" or "B. Special Payment Instructions" of this Letter of Transmittal, or for the account of an Eligible Institution. If the Notes are registered in the name of a person other than the signer of this Letter of Transmittal, or if certificates for unpurchased Notes are to be issued to a person other than the registered holder, the signatures on this Letter of Transmittal accompanying the tendered Notes must be guaranteed by a Medallion Signature Guarantor as described above. See Instruction 5.

        2.    Delivery of Letter of Transmittal and Notes; Guaranteed Delivery Procedures.    This Letter of Transmittal is to be completed by holders if:

•

certificates for tendered Notes are to be physically delivered to the Depositary;



•

certificates for tendered Notes are being delivered pursuant to a Notice of Guaranteed Delivery previously sent to the Depositary; or



•

tender of Notes is to be made by book-entry transfer to the Depositary's account at DTC pursuant to the procedures for book-entry transfer set forth under the caption "Terms of the Offer—Procedure for Tendering" in the Statement, and instructions are not being transmitted through ATOP.

All physically delivered Notes, or a confirmation of a book-entry transfer into the Depositary's account at DTC of all Notes delivered electronically, as well as a properly completed and duly executed Letter of Transmittal (or a facsimile thereof) or an Agent's Message and any other documents required by this Letter of Transmittal, must be received by the Depositary at its address set forth herein on or before the Expiration Date (in order to receive the purchase price).

        DELIVERY OF DOCUMENTS TO DTC DOES NOT CONSTITUTE DELIVERY TO THE DEPOSITARY. DELIVERY WILL BE DEEMED MADE ONLY WHEN DOCUMENTS ARE ACTUALLY RECEIVED BY THE DEPOSITARY.

        The method of delivery of this Letter of Transmittal, certificates for Notes and all other required documents, including delivery through DTC and any acceptance or Agent's Message delivered through ATOP, is at the election and risk of the tendering holder. If a holder chooses to deliver by mail, the recommended method is by registered mail with return receipt requested, properly insured. In all cases, sufficient time should be allowed to ensure timely delivery to the Depositary.

        Holders whose certificates are not immediately available or who cannot deliver certificates for their Notes and all other required documents to the Depositary on or before the Expiration Date, or whose Notes cannot be delivered on or before the Expiration Date under the procedures for book-entry transfer, may tender their Notes by or through any Eligible Institution by properly completing and duly executing and delivering a Notice of Guaranteed Delivery, or facsimile of it, and by otherwise complying with the guaranteed delivery procedure set forth in the section of the Statement titled "Terms of the Offer—Procedure for Tendering Notes—Guaranteed Delivery" of the Statement. Under such procedure, the certificates for all physically-tendered Notes or book-entry confirmation, as the case may be, as well as a properly completed and duly executed Letter of Transmittal, or manually signed facsimile of it, or an Agent's Message, and all other documents required by this Letter of Transmittal, must be received by the Depositary within three New York Stock Exchange trading days after receipt by the Depositary of such Notice of Guaranteed Delivery, all as provided in the section of the Statement titled "Terms of the Offer—Procedure for Tendering Notes—Guaranteed Delivery."

        The Notice of Guaranteed Delivery may be delivered by hand, mail, overnight courier, telegram or facsimile transmission to the Depositary and must include, if necessary, a guarantee by an Eligible Institution in the form set forth in such notice. For Notes to be tendered validly under the guaranteed delivery procedure, the Depositary must receive the Notice of Guaranteed Delivery on or before the Expiration Date.

8

        No alternative, conditional or contingent tenders will be accepted. All tendering holders, by execution of this Letter of Transmittal, or a facsimile hereof, waive any right to receive any notice of the acceptance of their Notes for payment.

        3.    Inadequate Space.    If the space provided in this Letter of Transmittal is inadequate, the certificate numbers and/or the principal amount represented by Notes should be listed on a separate signed schedule attached to this Letter of Transmittal.

        4.    Partial Tenders.    (Not applicable to holders who tender by book-entry transfer.) Tenders of Notes will be accepted only in integral multiples of $1,000 principal amount. If holders wish to tender with respect to less than the entire principal amount of any Notes submitted, holders must fill in the principal amount that is to be tendered in the columns entitled "Principal Amount Tendered." In the case of a partial tender of Notes, as soon as practicable after the Expiration Date, new certificates for the remainder of the Notes that were evidenced by such holder's old certificates will be sent to such holder, unless otherwise provided in the appropriate box of this Letter of Transmittal. The entire principal amount that is represented by the Notes delivered to the Depositary will be deemed to have been tendered, unless otherwise indicated.

        5.    Signature on Letter of Transmittal, Instruments of Transfer and Endorsements.    If this Letter of Transmittal is signed by the registered holders tendered hereby the signatures must correspond with the name(s) as written on the face of the certificate(s) without alteration, enlargement or any change whatsoever. If this Letter of Transmittal is signed by a participant in DTC whose name is shown as the owner of the Notes tendered hereby, the signature must correspond with the name shown on the Note position listing as the owner of the Notes.

        If any of the Notes tendered hereby are registered in the name of two or more holders, all such holders must sign this Letter of Transmittal. If any of the Notes tendered hereby are registered in different names on several certificates, it will be necessary to complete, sign and submit as many separate Letters of Transmittal as there are different registrations of certificates.

        If this Letter of Transmittal or any Note or instrument of transfer is signed by a trustee, executor, administrator, guardian, attorney-in-fact, agent, officer of a corporation or other person acting in a fiduciary or representative capacity, such person should so indicate when signing, and proper evidence satisfactory to Cephalon of such person's authority to so act must be submitted.

        When this Letter of Transmittal is signed by the registered holders of the Notes listed and transmitted hereby, no endorsements of Notes or separate instruments of transfer are required unless payment is to be made, or Notes not tendered or purchased are to be issued, to a person other than the registered holders, in which case signatures on such Notes or instruments of transfer must be guaranteed by a Medallion Signature Guarantor.

        If this Letter of Transmittal is signed other than by the registered holders of the Notes listed, the Notes must be endorsed or accompanied by appropriate instruments of transfer entitling the undersigned to tender the Notes on behalf of such registered holders, in any case signed exactly as the name or names of the registered holders appear on the Notes, with the signatures on the certificates or instruments of transfer guaranteed by a Medallion Signature Guarantor, unless the signature is that of an Eligible Institution.

        6.    Special Issuance/Delivery and Special Payment Instructions.    If a check and/or certificates for Notes representing principal amounts not tendered or not accepted for payment are to be issued in the name of a person other than the signer of this Letter of Transmittal, or if a check is to be sent and/or such Notes are to be returned to someone other than the signer of this Letter of Transmittal or to an address other than that shown above, the appropriate "A. Special Issuance/Delivery Instructions" or "B. Special Payment Instructions" box on this Letter of Transmittal should be completed. All Notes tendered by book-entry transfer and not accepted for payment will, except as otherwise indicated, be returned by crediting the account at DTC designated above as the account for which such Notes were delivered.

        7.    Transfer Taxes.    Except as set forth in this Instruction 7, Cephalon will pay or cause to be paid any transfer taxes with respect to the transfer and sale of Notes to Cephalon, or to Cephalon's order, pursuant to the Offer. If payment is to be made to, or if Notes not tendered or purchased are to be registered in the name of, any persons other than the registered owners, or if tendered Notes are registered in the name of any persons other than the persons signing this Letter of Transmittal, the amount of any transfer taxes (whether imposed on the registered holder or such other person) payable on account of the transfer to such

9

other person will be deducted from the payment unless satisfactory evidence of the payment of such taxes or exemption therefrom is submitted.

        8.    Waiver of Conditions.    The conditions of the Offer are for the sole benefit of Cephalon. The conditions may be asserted by Cephalon regardless of the circumstances, including any action or inaction by Cephalon, giving rise to such condition or may be waived by Cephalon in whole or in part at any time and from time to time in its sole discretion on or prior to the Expiration Date. The failure of Cephalon at any time to exercise any of its rights will not be deemed a waiver of any other right and each right will be deemed an ongoing right which may be asserted at any time and from time to time.

        9.    Substitute Form W-9 and Form W-8.    Each tendering holder must either (i) provide the Depositary with a correct taxpayer identification number ("TIN"), generally the holder's social security or federal employer identification number, and with certain other information, on Substitute Form W-9, which is provided under "Important Tax Information" below, and certify, under penalty of perjury, that such TIN is correct, such holder is not subject to backup withholding and such holder is a United States person or (ii) establish another basis for exemption from backup withholding. A tendering holder must cross out item (2) in the Certification box on Substitute Form W-9 if such holder is subject to backup withholding. Failure to provide the information on the Substitute Form W-9 may subject the tendering holder (or other payee) to a $50 penalty imposed by the Internal Revenue Service and a federal income tax backup withholding (currently 28%) on any payment made pursuant to the Offer. Certain holders (including, among others, all corporations and certain foreign individuals) are not subject to backup withholding and reporting requirements. A foreign person (including a foreign corporation) may qualify as an exempt recipient by submitting to the Depositary a properly completed Internal Revenue Service Form W-8, signed under penalties of perjury, attesting to that holder's exempt status. A Form W-8 can be obtained from the Depositary. See the enclosed "Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9" for additional instructions.

        10.    Requests for Assistance or Additional Copies.    Any questions or requests for assistance or additional copies of the Statement or this Letter of Transmittal may be directed to the Dealer Manager or the Information Agent at their respective addresses and/or telephone numbers set forth below. A holder may also contact such holder's broker, dealer, commercial bank or trust company or nominee for assistance concerning the Offer.

        IMPORTANT:    This Letter of Transmittal, or a facsimile hereof, together with Notes and all other required documents, must be received by the Depositary on or before the Expiration Date in order for holders to receive the purchase price.

IMPORTANT TAX INFORMATION

        Under U.S. federal income tax law, a holder whose tendered Notes are accepted for payment is generally required to provide the Depositary with such holder's correct TIN on Substitute Form W-9 below or otherwise establish a basis for exemption from backup withholding. A TIN is generally an individual holder's social security number or a holder's employer identification number. If the Depositary is not provided with the correct TIN, the holder may be subject to a $50 penalty imposed by the Internal Revenue Service. In addition, any payment made to such holder with respect to Notes purchased pursuant to the Offer may be subject to backup withholding (currently set at 28% of the payment). More serious penalties may be imposed for providing false information, which, if willfully done, may result in fines and/or imprisonment.

        Backup withholding is not an additional tax. Rather, the U.S. federal income tax liability of persons subject to backup withholding will be reduced by the amount of tax withheld. If withholding results in an overpayment of taxes, a refund may be obtained provided that the required information is furnished to the Internal Revenue Service.

        Certain holders (including, among others, corporations and certain foreign individuals) are not subject to these backup withholding and reporting requirements. Foreign holders must establish their status as exempt recipients from backup withholding and can do so by submitting to the Depositary a properly completed Internal Revenue Service Form W-8 (available from the Depositary), signed, under penalties of perjury, attesting to such holder's exempt foreign status.

10

        Holders are urged to consult their own tax advisors to determine whether they are exempt from backup withholding.

PURPOSE OF SUBSTITUTE FORM W-9

        To prevent backup withholding on any payment made to a holder with respect to Notes purchased pursuant to the Offer, the holder is required to notify the Depositary of the holder's correct TIN by completing the form below, certifying that: (A) the TIN provided on Substitute Form W-9 is correct; (B) either (1) the holder is exempt from backup withholding, (2) the holder has not been notified by the Internal Revenue Service that such holder is subject to backup withholding as a result of failure to report all interest or dividends or (3) the Internal Revenue Service has notified the holder that such holder is no longer subject to backup withholding; and (C) the holder is a United States person (including a United States resident alien).

WHAT NUMBER TO GIVE THE DEPOSITARY

        The holder is required to give the Depositary the TIN of the record owner of the Notes. If the Notes are registered in more than one name or are not registered in the name of the actual owner, consult the enclosed "Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9" for additional guidance on which number to report.

11

PAYER'S NAME: U.S. BANK NATIONAL ASSOCIATION, AS DEPOSITARY
SUBSTITUTE Form W-9 Department of the Treasury, Internal Revenue Service	Part 1—PLEASE PROVIDE YOUR TIN IN THE BOX AT RIGHT AND CERTIFY BY SIGNING AND DATING BELOW.	TIN (Social Security Number or Employer Identification Number)
	Part 2—Certification—Under penalties of perjury, I certify that:
	(1) The number shown on this form is my correct Taxpayer Identification Number;
Payer's Request for Taxpayer Identification Number (TIN)	(2) I am not subject to backup withholding because (a) I am exempt from backup withholding, (b) I have not been notified by the Internal Revenue Service ("IRS") that I am subject to backup withholding as a result of failure to report all interest or dividends, or (c) the IRS has notified me that I am not longer subject to backup withholding; and (3) I am a U.S. person (including a U.S. resident alien).

Certificate Instructions—You must cross out item (2) in Part 2 above if you have been notified by the IRS that you are subject to backup withholding because of underreporting interest or dividends on your tax return.

Name:

Address:

Signature:

Date:

NOTE:

FAILURE TO COMPLETE AND RETURN THIS FORM MAY RESULT IN BACKUP WITHHOLDING OF 28% OF THE PAYMENT MADE TO YOU PURSUANT TO THE OFFER. PLEASE REVIEW THE ENCLOSED GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9 FOR ADDITIONAL DETAILS.

12

        Questions and requests for assistance may be directed to Deutsche Bank Securities Inc. (the "Dealer Manager") or Morrow & Co., Inc. (the "Information Agent"). Requests for additional copies of the Statement or this Letter of Transmittal should be directed to the Information Agent.

The Depositary for the Offer is:

U.S. Bank National Association

West Side Flats Operations Center
60 Livingston Avenue
St. Paul, MN 55107
Attn: Specialized Finance
For Information: (800) 934-6802

By Facsimile:
(651) 495-8158
Attn: Specialized Finance
Confirm by Receipt of Facsimile Only: (651) 495-3511

The Information Agent for the Offer is:

445 Park Avenue, 5th Floor
New York, New York 10022
Email: ceph.info@morrowco.com
Banks and Brokerage Firms, Please Call (800) 654-2468
Noteholders, Please Call (800) 607-0088
All Others Call Collect (212) 754-8000

The Dealer Manager for the Offer is:

Deutsche Bank Securities

60 Wall Street
New York, New York 10005

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Exhibit 99(a)(1)(ii)

EX-99.A1III

Exhibit 99(a)(1)(iii)

CEPHALON, INC.

Offer to Purchase for Cash
any and all of the Outstanding
2¹/2% Convertible Subordinated Notes Due December 2006
of Cephalon, Inc.

(CUSIP Nos. 156708AD1 and 156708AE9)

THE OFFER AND YOUR WITHDRAWAL RIGHTS WILL EXPIRE AT 5 P.M., NEW YORK CITY TIME, ON JULY 11, 2005, UNLESS THE OFFER IS EXTENDED.

June 10, 2005

To: Brokers, Dealers, Commercial Banks, Trust Companies and other Nominees:

        Enclosed for your consideration is an Offer to Purchase (as it may be amended or supplemented from time to time, the "Statement") and a form of Letter of Transmittal (as it may be amended or supplemented from time to time, the "Letter of Transmittal" and, together with the Statement, the "Offer"), relating to the offer by Cephalon, Inc., a Delaware corporation ("Cephalon"), to purchase for cash any and all of its outstanding 2¹/2% Convertible Subordinated Notes due December 2006 at a price of $975.00 for each $1,000 principal amount of Notes purchased pursuant to the Offer, plus accrued and unpaid interest to, but not including, the date of payment for the Notes accepted for payment. Capitalized terms used in this letter and not defined in this letter will have the meanings ascribed to them in the Statement.

        We have been engaged by Cephalon to act as the Dealer Manager in connection with the Offer.

        The Offer is being made upon the terms and subject to the conditions set forth in the Statement and in the related Letter of Transmittal.

        For your information and for forwarding to your clients for whom you hold Notes registered in your name or in the name of your nominee, we are enclosing the following documents:

1.

The Offer to Purchase dated June 10, 2005;



2.

A form of Letter of Transmittal for the Notes for your use and for the information of your clients;



3.

A form of Notice of Guaranteed Delivery to be used to accept the Offer if certificates for Notes are not immediately available or cannot be delivered to U.S. Bank National Association, the Depositary for the Offer, on or before the Expiration Date, or if the procedure for book-entry transfer cannot be completed on or before the Expiration Date, or if time will not permit all required documents to reach the Depositary on or prior to the Expiration Date;



4.

A letter which may be sent to your clients for whose accounts you hold Notes registered in your name or in the name of your nominee, with space provided for obtaining such clients' instructions with regard to the Offer; and



5.

The Guidelines for Certification of Taxpayer Identification Number on Substitute Form W-9.

        Participants of The Depository Trust Company ("DTC") will be able to execute tenders through the DTC Automated Tender Offer Program.

        WE URGE YOU TO CONTACT YOUR CLIENTS AS PROMPTLY AS POSSIBLE TO OBTAIN THEIR INSTRUCTIONS.

        Any inquiries you may have with respect to the Offer should be addressed to Deutsche Bank Securities Inc., the Dealer Manager for the Offer, at 60 Wall Street, New York, New York 10005. You may also contact Morrow & Co., Inc., the Information Agent for the Offer, at the address and telephone numbers as set forth on the back cover page of the enclosed Statement. Additional copies of the enclosed material may be obtained from the Information Agent.

Very truly yours,

DEUTSCHE BANK SECURITIES INC.

        NOTHING CONTAINED HEREIN OR IN THE ENCLOSED DOCUMENTS WILL CONSTITUTE YOU AS THE AGENT OF CEPHALON, THE DEALER MANAGER, THE DEPOSITARY OR THE INFORMATION AGENT, OR AUTHORIZE YOU TO GIVE ANY INFORMATION OR TO MAKE ANY REPRESENTATIONS OTHER THAN AS CONTAINED HEREIN OR IN THE STATEMENT OR THE LETTER OF TRANSMITTAL AND, IF GIVEN OR MADE, SUCH INFORMATION OR REPRESENTATIONS MUST NOT BE RELIED UPON AS HAVING BEEN AUTHORIZED BY CEPHALON, THE DEALER MANAGER, THE DEPOSITARY OR THE INFORMATION AGENT. THE STATEMENT AND RELATED DOCUMENTS DO NOT CONSTITUTE AN OFFER TO BUY OR SOLICITATION OF AN OFFER TO SELL NOTES IN ANY CIRCUMSTANCES IN WHICH SUCH OFFER OR SOLICITATION IS UNLAWFUL.

2

EX-99.A1IV

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Exhibit 99(a)(1)(iv)

CEPHALON, INC.

Offer to Purchase for Cash
any and all of the Outstanding
2¹/2% Convertible Subordinated Notes Due December 2006
of Cephalon, Inc.

(CUSIP Nos. 156708AD1 and 156708AE9)

THE OFFER AND YOUR WITHDRAWAL RIGHTS WILL EXPIRE AT 5 P.M., NEW YORK CITY TIME, ON JULY 11, 2005, UNLESS THE OFFER IS EXTENDED.

        June 10, 2005

To Our Clients:

        Enclosed for your consideration is an Offer to Purchase (as it may be amended or supplemented from time to time, the "Statement") and a form of Letter of Transmittal (as it may be amended or supplemented from time to time, the "Letter of Transmittal" and, together with the Statement, the "Offer"), relating to the offer by Cephalon, Inc., a Delaware corporation ("Cephalon"), to purchase for cash any and all of its outstanding 2¹/2% Convertible Subordinated Notes due December 2006 (the "Notes") at a price of $975.00 for each $1,000 principal amount of Notes purchased pursuant to the Offer, plus accrued and unpaid interest to, but not including, the date of payment for the Notes accepted for payment. Capitalized terms used in this letter and not defined in this letter have the meanings ascribed to them in the Statement.

        The material relating to the Offer is being forwarded to you as the beneficial owner of Notes carried by us for your account or benefit but not registered in your name. A tender of any Notes may be made only by us as the registered holder of the Notes and pursuant to your instructions. Therefore, beneficial owners of Notes registered in the name of a broker, dealer, commercial bank, trust company or other nominee are urged to contact and instruct that broker, dealer, commercial bank, trust company or other nominee if such beneficial owner desires to tender those Notes pursuant to the Offer.

        Accordingly, we request your instructions as to whether you wish us to tender any or all of the Notes held by us for your account. We urge you to read carefully the Statement, the Letter of Transmittal and the other materials provided with this letter before instructing us to tender your Notes.

        Your instructions to us should be forwarded as promptly as possible in order to permit us to tender Notes on your behalf in accordance with the provisions of the Statement. The Offer and withdrawal rights will expire at 5 p.m., New York City time, on July 11, 2005, the Expiration Date, unless the Offer is extended. To receive the purchase price, valid tenders must be received by the Depositary on or before the Expiration Date.

        Your attention is directed to the following:

1.

The Offer is for any and all Notes that are outstanding.



2.

If you desire to tender any Notes pursuant to the Offer and to receive the purchase price, we must receive your instructions in ample time to permit us to effect a tender of Notes on your behalf on or before the Expiration Date.



3.

The Offer is being made upon the terms and subject to the conditions set forth in the Statement and in the related Letter of Transmittal.



4.

Any transfer taxes incident to the transfer of Notes from the tendering holder to Cephalon will be paid by Cephalon, except as provided in Instruction 7 to the Letter of Transmittal.

        If you wish to have us tender any or all of your Notes held by us for your account or benefit pursuant to the Statement, please so instruct us by completing, executing and returning to us the instruction form that appears below. THE ACCOMPANYING LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR INFORMATIONAL PURPOSES ONLY AND MAY NOT BE USED BY YOU TO TENDER NOTES HELD BY US AND REGISTERED IN OUR NAME FOR YOUR ACCOUNT.

INSTRUCTIONS

        The undersigned acknowledge(s) receipt of your letter and the enclosed material referred to therein relating to the Offer by Cephalon with respect to the Notes.

        THIS WILL INSTRUCT YOU TO TENDER THE PRINCIPAL AMOUNT OF NOTES INDICATED BELOW HELD BY YOU FOR THE ACCOUNT OR BENEFIT OF THE UNDERSIGNED, PURSUANT TO THE TERMS OF, AND CONDITIONS SET FORTH IN, THE OFFER TO PURCHASE DATED JUNE 10, 2005 AND THE LETTER OF TRANSMITTAL.

Certificate No.

Principal Amount at Maturity Tendered*                  

*

If no amount is indicated, the undersigned will be deemed to have instructed you to tender the entire principal amount at maturity of Notes held by the undersigned.

PLEASE SIGN HERE:

Name(s):

PLEASE PRINT HERE:

Address:

Area Code and Telephone No.:

Employer Identification or Social Security No.:

My Account Number With You:

Date:

2

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Exhibit 99(a)(1)(iv)

EX-99.A1V

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Exhibit 99(a)(1)(v)

NOTICE OF GUARANTEED DELIVERY

CEPHALON, INC.

For Tender of
2¹/2% Convertible Subordinated Notes Due December 2006
Pursuant to its Offer
Described in the Offer to Purchase dated June 10, 2005

THE OFFER AND YOUR WITHDRAWAL RIGHTS WILL EXPIRE AT 5 P.M., NEW YORK CITY TIME, ON JULY 11, 2005, UNLESS THE OFFER IS EXTENDED.

The Depositary for the Offer is:
U.S. Bank National Association

By Mail, Overnight Mail, Courier or Hand:
U.S. Bank National Association
West Side Flats Operations Center
60 Livingston Avenue
St. Paul, MN 55107
Attn: Specialized Finance
For Information: (800) 934-6802

By Facsimile:
(651) 495-8158
Attn: Specialized Finance
Confirm by Receipt of Facsimile Only: (651) 495-3511

DELIVERY OF THIS NOTICE OF GUARANTEED DELIVERY TO AN ADDRESS OTHER THAN AS SET FORTH ABOVE, OR TRANSMISSION TO A FACSIMILE NUMBER OTHER THAN AS SET FORTH ABOVE, WILL NOT CONSTITUTE A VALID DELIVERY.

        You must use this form to accept the offer of Cephalon, Inc. (the "Company") made pursuant to the Offer to Purchase dated June 10, 2005 (as it may be amended or supplemented from time to time, the "Statement"), and the related Letter of Transmittal (as it may be amended or supplemented from time to time, the "Letter of Transmittal" and, together with the Statement, the "Offer"), if the certificates for your 2¹/2% Convertible Subordinated Notes due December 2006 (the "Notes") are not immediately available or cannot be delivered to U.S. Bank National Association, the depositary for the Offer (the "Depositary"), on or before the Expiration Date, or the procedure for book-entry transfer cannot be completed on or before the Expiration Date or if time will not permit all required documents to reach the Depositary on or prior to the Expiration Date. This Notice of Guaranteed Delivery may be delivered or transmitted by facsimile transmission, mail or hand delivery to the Depositary as set forth below. In addition, in order to utilize the guaranteed delivery procedures to tender your Notes pursuant to the Offer, a Letter of Transmittal (or manually signed facsimile thereof) or an electronic confirmation pursuant to The Depository Trust Company's Automated Tender Offer Program ("ATOP") system, with any required signature guarantees and any other required documents (including an Agent's Message, or an express acknowledgment, confirming that you have received and agree to be bound by the Letter of Transmittal and that the Letter of Transmittal may be enforced against you), must also be received by the Depositary on or prior to the Expiration Date. Capitalized terms used but not defined herein are defined in the Statement.

THIS NOTICE OF GUARANTEED DELIVERY IS NOT TO BE USED TO GUARANTEE SIGNATURES. IF A SIGNATURE ON A LETTER OF TRANSMITTAL IS REQUIRED TO BE GUARANTEED BY AN "ELIGIBLE INSTITUTION" UNDER THE INSTRUCTIONS THERETO, SUCH SIGNATURE GUARANTEE MUST APPEAR IN THE APPLICABLE SPACE PROVIDED ON THE LETTER OF TRANSMITTAL.

Ladies and Gentlemen:

        Upon the terms and conditions set forth in the Statement and the accompanying Letter of Transmittal, the undersigned hereby tenders to the Company the aggregate principal amount of Notes set forth below pursuant to the guaranteed delivery procedures described in the section entitled "Terms of the Offer—Procedure for Tendering Notes—Guaranteed Delivery" in the Statement. By so tendering, the undersigned does hereby make, at and as of the date hereof, the representations and warranties of a tendering holder of Notes set forth in the Letter of Transmittal.

        All authority herein conferred or agreed to be conferred by this Notice of Guaranteed Delivery shall survive the death or incapacity of the undersigned and every obligation of the undersigned hereunder shall be binding upon the heirs, personal representatives, executors, successors, assigns, trustees in bankruptcy and other legal representatives of the undersigned.

PLEASE SIGN AND COMPLETE

Certificate Numbers of Notes
(if Available) Principal Amount of
Notes Tendered

Signature(s) of Registered Holder(s) or Authorized Signatory

Name(s):
	(Please Type or Print)
Title:
Address:

Area Code and Telephone Number:

Date:

If Notes will be delivered by book-entry transfer, check the box below and provide the account number requested:

o

The Depository Trust Company

Account Number:

        This Notice of Guaranteed Delivery must be signed by the holder(s) of Notes exactly as its (their) name(s) appear(s) on a security position listing as the owner of the Notes, or by person(s) authorized to become registered holder(s) by endorsements and documents transmitted with this Notice of Guaranteed Delivery. If signature is by an attorney-in-fact, trustee, executor, administrator, guardian, officer or other person acting in a fiduciary or representative capacity, such person must provide the following information:

Name: Capacity: Address(es):

Do not send Notes with this form. Notes should be sent to the Depositary together with a properly completed and duly executed Letter of Transmittal.

GUARANTEE
(Not To Be Used For Signature Guarantee)

        The undersigned, a firm that is a participant in the Securities Transfer Agents Medallion Program or an "Eligible Guarantor Institution" (as such term is defined in Rule 17Ad-15 under the Securities Exchange Act of 1934, as amended), hereby guarantees that, within three New York Stock Exchange trading days after the date of execution of this Notice of Guaranteed Delivery, a properly completed and duly executed Letter of Transmittal (or manually signed facsimile thereof), the Notes, in proper form for transfer, or a book-entry confirmation of transfer of such Notes into the Depositary's account at The Depository Trust Company, including the Agent's Message instead of a Letter of Transmittal, as the case may be, with any required signature guarantees and any other documents required by the Letter of Transmittal, will be deposited by the undersigned with the Depositary.

THE UNDERSIGNED ACKNOWLEDGES THAT IT MUST DELIVER THE LETTER OF TRANSMITTAL AND THE NOTES TENDERED HEREBY, OR, IN THE CASE OF A BOOK-ENTRY TRANSFER, AN AGENT'S MESSAGE INSTEAD OF A LETTER OF TRANSMITTAL, TO THE DEPOSITARY WITHIN THE TIME PERIOD SET FORTH ABOVE AND THAT FAILURE TO DO SO COULD RESULT IN FINANCIAL LOSS TO THE UNDERSIGNED.

SIGN HERE

Name of Firm:

Authorized Signature:

Name and Title (please type or print):

Address:

Area Code and Telephone Number:

Date:

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Exhibit 99(a)(1)(v)

NOTICE OF GUARANTEED DELIVERY

EX-99.A1VI

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Exhibit 99(a)(1)(vi)

GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION
NUMBER ON SUBSTITUTE FORM W-9

GUIDELINES FOR DETERMINING THE PROPER IDENTIFICATION NUMBER TO GIVE THE PAYER.—Social Security numbers have nine digits separated by two hyphens: i.e. 000-00-0000. Employer identification numbers have nine digits separated by only one hyphen: i.e. 00-0000000. The table below will help determine the number to give the payer.

For this type of account:			Give the SOCIAL SECURITY number of—
1.	Individual		The individual
2.	Two or more individuals (joint account)		The actual owner of the account or, if combined funds, the first individual on the account(1)
3.	Custodian account of a minor (Uniform Gift to Minors Act)		The minor(2)
4.	a.	The usual revocable savings trust (grantor is also trustee)	The grantor-trustee(1)
	b.	So-called trust account that is not a legal or valid trust under state law	The actual owner(1)
5.	Sole proprietorship		The owner(3)
For this type of account:			Give the EMPLOYER IDENTIFICATION number of—
6.	A valid trust, estate, or pension trust		The legal entity (Do not furnish the identifying number of the personal representative or trustee unless the legal entity itself is not designated in the account title.)(4)
7.	Corporate		The corporation
8.	Association, club, religion, charitable, educational, or other tax-exempt organization		The organization
9.	Partnership		The partnership
10.	A broker or registered nominee		The broker or nominee
11.	Account with the Department of Agriculture in the name of a public entity (such as a state or local government, school district, or prison) that receives agricultural program payments		The public entity

(1)

List first and circle the name of the person whose number you furnish.

(2)

Circle the minor's name and furnish the minor's social security number.

(3)

Show the individual's name. You may also enter your business name. You may use your Social Security number or Employer Identification number.

(4)

List first and circle the name of the legal trust, estate, or pension trust.

NOTE:  If no name is circled when there is more than one name, the number will be considered to be that of the first name listed.

GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION
NUMBER ON SUBSTITUTE FORM W-9
Page 2

Section references are to the Internal Revenue Code of 1986, as amended

Obtaining a Number

If you don't have a taxpayer identification number or you don't know your number, obtain Form SS-5, Application for a Social Security Number Card, Form W-7, Application for IRS Individual Taxpayer Identification Number or Form SS-4, Application for Employer Identification Number, at the local office of the Social Security Administration or the Internal Revenue Service (the "IRS") and apply for a number.

Payees Exempt from Backup Withholding

The following is a list of payees that may be exempt from backup withholding.

         (1)  A corporation.

         (2)  An organization exempt from tax under section 501(a), or an individual retirement plan ("IRA"), or a custodial account under section 403(b)(7) if the account satisfies the requirements of section 401(f)(2).

         (3)  The United States or any of its agencies or instrumentalities.

         (4)  A State, the District of Columbia, a possession of the United States, or any of their political subdivisions or instrumentalities.

         (5)  A foreign government or any of its political subdivisions, agencies or instrumentalities.

         (6)  An international organization or any of its agencies or instrumentalities.

         (7)  A foreign central bank of issue.

         (8)  A dealer in securities or commodities required to register in the United States or a possession of the United States.

         (9)  A futures commission merchant registered with the Commodity Futures Trading Commission.

       (10)  A real estate investment trust.

       (11)  An entity registered at all times during the tax year under the Investment Company Act of 1940.

       (12)  A common trust fund operated by a bank under section 584(a).

       (13)  A financial institution.

       (14)  A middleman known in the investment community as a nominee or custodian.

       (15)  A trust exempt from tax under section 664 or described in section 4947.

        Payments of dividends generally not subject to backup withholding include the following:

•

Payments to nonresident aliens subject to withholding under section 1441.

•

Payments to partnerships not engaged in a trade or business in the United States and that have at least one nonresident alien partner.

•

Payments made by certain foreign organizations.

        Payments of interest not generally subject to backup withholding include the following:

•

Payments of tax-exempt interest (including exempt interest dividends under section 852).

•

Payments described in section 6049(b)(5) to nonresident aliens.

•

Payments on tax-free covenant bonds under section 1451.

•

Payments made by certain foreign organizations.

Payments that are not subject to information reporting are also not subject to backup withholding. For details see sections 6041, 6041A, 6042, 6044, 6045, 6049, 6050A, and 6050N, and the regulations under such sections. Exempt payees should complete a Substitute Form W-9 to avoid possible erroneous backup withholding.

Privacy Act Notice.—Section 6109 requires you to give your correct taxpayer identification number to persons who must file information returns with the IRS to report interest, dividends, and certain other income paid to you, mortgage interest you paid, the acquisition or abandonment of secured property, cancellation of debt, or contributions you made to an IRA. The IRS uses the numbers for identification purposes and to help verify the accuracy of your tax return and may also provide this information to various government agencies for tax enforcement or litigation purposes. The IRS may also disclose this information to other countries under a tax treaty. You must provide your taxpayer identification number whether or not you are required to file a tax return. Payers must generally withhold 28% of taxable interest, dividend, and certain other payments to a payee who does not furnish a taxpayer identification number to a payer. Certain penalties may also apply.

Penalties

(1)  Penalty for Failure to Furnish Taxpayer Identification Number.—If you fail to furnish your taxpayer identification number to a payer, you are subject to a penalty of $50 for each such failure unless your failure is due to reasonable cause and not to willful neglect.

(2)  Civil Penalty for False Information With Respect to Withholding.—If you make a false statement with no reasonable basis that results in no backup withholding, you are subject to a $500 penalty.

(3)  Criminal Penalty for Falsifying Information.—Willfully falsifying certifications or affirmations may subject you to criminal penalties including fines and/or imprisonment.

FOR ADDITIONAL INFORMATION CONTACT YOUR TAX CONSULTANT OR THE IRS

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GUIDELINES FOR CERTIFICATION OF TAXPAYER IDENTIFICATION NUMBER ON SUBSTITUTE FORM W-9

EX-99.A5

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Exhibit 99(a)(5)

News

Investor Contact: Chip Merritt
610-738-6376
cmerritt@cephalon.com

Media Contact: Robert W. Grupp
610-738-6402
rgrupp@cephalon.com

For Immediate Release

Cephalon Commences Tender Offer for 2¹/2 Percent
Convertible Subordinated Notes Due 2006

Frazer, PA—June 10, 2005—Cephalon, Inc. (Nasdaq: CEPH) announced today that it has commenced a cash tender offer for all of its outstanding 2¹/2 percent Convertible Subordinated Notes Due December 2006. The tender offer will expire at 5:00 p.m. New York City Time on July 11, 2005, unless extended. The tender offer, which is described fully in the offer to purchase and the related letter of transmittal, is not subject to the receipt of any minimum amount of tenders.

        Cephalon is purchasing the notes to reduce outstanding debt and reduce interest expense. The tender offer will be funded from a portion of the proceeds of the company's recently completed public offering of 2.00 percent Senior Subordinated Convertible Notes due June 1, 2015. Cephalon is offering to purchase the notes at a price of $975.00 for each $1,000 of principal amount of notes tendered, plus accrued and unpaid interest up to, but not including, the date the notes are paid pursuant to the offer.

        Cephalon has retained Deutsche Bank Securities Inc. to act as Dealer Manager in connection with the offer.

        U.S. Bank National Association has been appointed to act as the depositary for the offer, and Morrow & Co., Inc. has been appointed to serve as information agent. Questions and requests for assistance and requests for copies of the offer to purchase and the related letter of transmittal may be directed to the information agent at 800-607-0088 or ceph.info@morrowco.com.

        Neither the Cephalon Board of Directors nor any other person makes any recommendation as to whether holders of notes should tender their notes, and no one has been authorized to make such a recommendation. Holders of notes must make their own decisions as to whether to tender their notes, and, if they decide to do so, the principal amount of notes to tender.

—more—

SOURCE: Cephalon, Inc.    •    41 Moores Road    •    Frazer, PA 19355    •    (610) 344-0200    •    Fax (610) 344-0981

Cephalon Commences Tender Offer for 2¹/2 Percent Convertible Subordinated Notes

        This announcement is neither an offer to buy nor a solicitation of an offer to sell any securities and shall not constitute an offer, solicitation or sale in any jurisdiction in which such offer, solicitation or sale is unlawful. The complete terms and conditions of the tender offer are set forth in the offer to purchase dated today and the related letter of transmittal, which are included in a Tender Offer Statement on Schedule TO that is being filed by Cephalon today with the Securities and Exchange Commission. The Tender Offer Statement (including the offer to purchase, the letter of transmittal and related documents) will contain important information which should be read carefully before any decision is made with respect to the tender offer. The offer to purchase and the related letter of transmittal are being delivered to holders of the notes. Cephalon's Tender Offer Statement (including the offer to purchase, the letter of transmittal and related documents) will also be available for free on the Commission's Web site at www.sec.gov.

        Cephalon, Inc.

        Cephalon currently markets three proprietary products in the United States: PROVIGIL® (modafinil) [C-IV], GABITRIL® (tiagabine hydrochloride) and ACTIQ® (oral transmucosal fentanyl citrate) [C-II] and more than 20 products internationally.

* * *

In addition to historical facts or statements of current condition, this press release may contain forward-looking statements. Forward-looking statements provide Cephalon's current expectations or forecasts of future events. These may include statements regarding anticipated scientific progress on its research programs, development of potential pharmaceutical products, interpretation of clinical results, prospects for regulatory approval, manufacturing development and capabilities, market prospects for its products, sales and earnings guidance, and other statements regarding matters that are not historical facts. You may identify some of these forward-looking statements by the use of words in the statements such as "anticipate," "estimate," "expect," "project," "intend," "plan," "believe" or other words and terms of similar meaning. Cephalon's performance and financial results could differ materially from those reflected in these forward-looking statements due to general financial, economic, regulatory and political conditions affecting the biotechnology and pharmaceutical industries as well as more specific risks and uncertainties facing Cephalon such as those set forth in its reports on Form 8-K, 10-Q and 10-K filed with the U.S. Securities and Exchange Commission. Given these risks and uncertainties, any or all of these forward-looking statements may prove to be incorrect. Therefore, you should not rely on any such factors or forward-looking statements. Furthermore, Cephalon does not intend to update publicly any forward-looking statement, except as required by law.

# # #

SOURCE: Cephalon, Inc.    •    41 Moores Road    •    Frazer, PA 19355    •    (610) 344-0200    •    Fax (610) 344-0981

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Exhibit 99(a)(5)