<?xml version="1.0"?><ownershipDocument><schemaVersion>X0201</schemaVersion>    <documentType>4</documentType><periodOfReport>2003-09-17</periodOfReport><notSubjectToSection16>0</notSubjectToSection16><issuer><issuerCik>0000057725</issuerCik><issuerName>LANNETT CO INC</issuerName>        <issuerTradingSymbol>LCI</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId><rptOwnerCik>0001255689</rptOwnerCik><rptOwnerName>FARBER WILLIAM</rptOwnerName></reportingOwnerId><reportingOwnerAddress><rptOwnerStreet1>32640 WHATLEY ROAD</rptOwnerStreet1><rptOwnerStreet2></rptOwnerStreet2><rptOwnerCity>FRANKLIN</rptOwnerCity><rptOwnerState>MI</rptOwnerState><rptOwnerZipCode>48025</rptOwnerZipCode><rptOwnerStateDescription></rptOwnerStateDescription></reportingOwnerAddress><reportingOwnerRelationship><isDirector>1</isDirector><isOfficer>1</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther><officerTitle>Chief Executive Officer</officerTitle></reportingOwnerRelationship></reportingOwner><nonDerivativeTable><nonDerivativeTransaction><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-09-17</value></transactionDate><deemedExecutionDate></deemedExecutionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionTimeliness><value></value></transactionTimeliness><transactionAmounts><transactionShares><value>7100</value></transactionShares><transactionPricePerShare><value>20</value></transactionPricePerShare><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>13578579</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership></ownershipNature></nonDerivativeTransaction></nonDerivativeTable><footnotes></footnotes><remarks></remarks><ownerSignature><signatureName>Larry Dalesandro</signatureName><signatureDate>2003-09-18</signatureDate></ownerSignature></ownershipDocument>

