<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-05-15</periodOfReport>

    <issuer>
        <issuerCik>0000038074</issuerCik>
        <issuerName>FOREST LABORATORIES INC</issuerName>
        <issuerTradingSymbol>FRX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001509230</rptOwnerCik>
            <rptOwnerName>Lynch Jerome</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O FOREST LABORATORIES, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>909 THIRD AVENUE</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10022</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>SVP-Sales</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-05-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>863</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.875</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>64567</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">These shares of common stock were issued under the Company's 2007 Equity Incentive Plan to the reporting person on May 7, 2012 as part of an annual grant of restricted stock.  The Form 4 filed with the SEC on May 9, 2012 which reported this grant inadvertently contained an incorrect vesting schedule.  The shares granted on May 7, 2012 are subject to a risk of forfeiture, which lapsed as to 33% of the shares on May 15, 2013, and lapses as to 33% of the shares on May 15, 2014 and 34% of the shares on May 15, 2015 (as opposed to lapsing on May 7, 2013, May 7, 2014 and May 7, 2015, respectively, as initially reported).</footnote>
        <footnote id="F2">This amount includes shares of common stock which are subject to a risk of forfeiture.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Jerome Lynch</signatureName>
        <signatureDate>2013-05-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
