EXHIBIT 5.1
                           MORGAN, LEWIS & BOCKIUS LLP
                               1701 Market Street
                             Philadelphia, PA 19103


February 14, 2002

Cephalon, Inc.
145 Brandywine Parkway
West Chester, PA 19380

Re:      Cephalon, Inc. - Registration Statement on Form S-3
         ---------------------------------------------------

Ladies and Gentlemen:

We have acted as counsel to Cephalon, Inc., a Delaware corporation (the
"Company"), in connection with the preparation and filing of the subject
Registration Statement on Form S-3 (the "Registration Statement") with the
Securities and Exchange Commission (the "Commission") under the Securities Act
of 1933, as amended (the "Act"), relating to the registration by the Company of
(i) $600,000,000 of principal amount of its 2 1/2% Subordinated Convertible
Notes due December 15, 2006 (the "Notes") and (ii) 7,407,480 shares of the
Company's common stock, par value $.01 per share, issuable upon the conversion
of the Notes (the "Conversion Shares"), all of which are to be sold by certain
holders of the Notes or the Conversion Shares as described in the Registration
Statement. The Notes were issued under an Indenture dated December 11, 2001 (the
"Indenture") by and between the Company and State Street Bank and Trust Company,
as trustee.

In rendering the opinion set forth below, we have examined (i) the Registration
Statement and the exhibits thereto, (ii) the Restated Certificate of
Incorporation of the Company, as amended, (the "Certificate"), (iii) the
Indenture, and (iv) the form of the Notes. We have also examined certain records
of the Company's corporate proceedings as reflected in its minute books and such
statutes, records, and other documents as we have deemed relevant. In our
examination, we have assumed the genuineness of documents examined by us, the
authenticity of all documents submitted to us as originals, the conformity with
the originals of all documents submitted to us as copies and the authenticity of
the originals of such latter documents.

Based on the foregoing, it is our opinion that (i) the Notes are legally issued
and binding obligations of the Company and (ii) the Conversion Shares have been
duly authorized by the Company and, when issued and delivered in accordance with
the terms of the Indenture, will be validly issued, fully paid and
nonassessable, assuming no change in applicable law.

The opinion set forth above is limited to the laws of the State of New York and
the General Corporation Law of the State of Delaware, as amended. We hereby
consent to the use of this opinion as Exhibit 5.1 to the Registration Statement.
In giving such consent, we do not thereby admit that we are acting within the
category of persons whose consent is required under Section 7 of the Act and the
rules or regulations of the Commission thereunder.


Very truly yours,


/s/ Morgan, Lewis & Bockius LLP