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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2015-04-01</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001009356</issuerCik>
        <issuerName>SALIX PHARMACEUTICALS LTD</issuerName>
        <issuerTradingSymbol>SLXP</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001314760</rptOwnerCik>
            <rptOwnerName>Forbes William P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>8510 COLONNADE CENTER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>RALEIGH</rptOwnerCity>
            <rptOwnerState>NC</rptOwnerState>
            <rptOwnerZipCode>27615</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President, Medical, R&amp;D &amp; CDO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2015-04-01</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>51714</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>173</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Pursuant to the terms of the Agreement and Plan of Merger, dated as of February 20, 2015 (as amended, the &quot;Merger Agreement&quot;), by and among the Issuer, Valeant Pharmaceuticals International (&quot;VPI&quot;), a Delaware corporation and a wholly owned subsidiary of Valeant, Sun Merger Sub, Inc. (&quot;Merger Sub&quot;), a Delaware corporation and a wholly owned subsidiary of VPI, and solely for purposes of guaranteeing VPI's and Merger Sub's obligations under the Merger Agreement, Valeant Pharmaceuticals International, Inc. (&quot;Valeant&quot;), a British Columbia corporation, on April 1, 2015, each share of the Issuer's restricted stock issued and outstanding immediately prior to the Effective Time (as defined in the Merger Agreement) was converted into the right to receive $173.00 per share in cash, subject to any applicable withholding taxes.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ William P. Forbes</signatureName>
        <signatureDate>2015-04-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
