-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 BKFXyYcgmMCr/lm7VIAq3Q==

<SEC-DOCUMENT>0001210193-03-000003.txt : 20031215
<SEC-HEADER>0001210193-03-000003.hdr.sgml : 20031215
<ACCEPTANCE-DATETIME>20031215180710
ACCESSION NUMBER:		0001210193-03-000003
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20031212
FILED AS OF DATE:		20031215

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ROCHE ROBERT P JR
		CENTRAL INDEX KEY:			0001210193

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-19119
		FILM NUMBER:		031055676

	MAIL ADDRESS:	
		STREET 1:		145 BRANDYWINE PARKWAY
		CITY:			WEST CHESTER
		STATE:			PA
		ZIP:			19380

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			CEPHALON INC
		CENTRAL INDEX KEY:			0000873364
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				232484489
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		145 BRANDYWINE PKWY
		CITY:			WEST CHESTER
		STATE:			PA
		ZIP:			19380
		BUSINESS PHONE:		6103440200

	MAIL ADDRESS:	
		STREET 1:		145 BRANDYWINE PARKWAY
		CITY:			WEST CHESTER
		STATE:			PA
		ZIP:			19380
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>

<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2003-12-12</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000873364</issuerCik>
        <issuerName>CEPHALON INC</issuerName>
        <issuerTradingSymbol>CEPH</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001210193</rptOwnerCik>
            <rptOwnerName>ROCHE ROBERT P JR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>145 BRANDYWINE PARKWAY</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WEST CHESTER</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19380</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Sr. VP, Pharmaceutical Ops.</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>

        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2003-12-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>818</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>47.5</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>17649</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>

        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2003-12-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1546</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>47.53</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>16103</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>

    </nonDerivativeTable>



    <ownerSignature>
        <signatureName>Carolyn S. Iosca, Attorney-in-Fact, For: Robert P. Roche, Jr.</signatureName>
        <signatureDate>2003-12-15</signatureDate>
    </ownerSignature>

</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----