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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2016-09-26</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001082038</issuerCik>
        <issuerName>DURECT CORP</issuerName>
        <issuerTradingSymbol>DRRX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001459708</rptOwnerCik>
            <rptOwnerName>21 APRIL FUND, LTD.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>CITCO FUND SERVICES (CAYMAN ISLANDS) LTD</rptOwnerStreet1>
            <rptOwnerStreet2>REGATTA OFFICE PARK WEST BAY ROAD</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAYMAN</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-1205</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001459688</rptOwnerCik>
            <rptOwnerName>21 APRIL FUND, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>FIRST EAGLE INVESTMENT MANAGEMENT, LLC</rptOwnerStreet1>
            <rptOwnerStreet2>1345 AVENUE OF THE AMERICAS, 44TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10105</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001457518</rptOwnerCik>
            <rptOwnerName>First Eagle Value in Biotechnology Master Fund, Ltd.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>CITCO FUND SERVICES (CAYMAN ISLANDS) LTD</rptOwnerStreet1>
            <rptOwnerStreet2>REGATTA OFFICE PARK WEST BAY ROAD</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAYMAN</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-1205</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001637141</rptOwnerCik>
            <rptOwnerName>First Eagle Holdings, Inc.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1345 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10105</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-09-26</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1981224</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1.16</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>21862770</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-09-26</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>318776</value>
                    <footnoteId id="F3"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1.10</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>22181546</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2016-09-26</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>200000</value>
                    <footnoteId id="F5"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>1.15</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>22381546</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F6"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Represents 259,957 and 1,721,267 shares purchased by 21 April Fund, LP (&quot;21 April LP&quot;) and 21 April Fund, Ltd. (&quot;21 April Ltd.&quot;), respectively. First Eagle Investment Management, LLC (&quot;FEIM&quot;) is (i) a general partner of 21 April LP, (ii) the registered investment adviser to First Eagle Value in Biotechnology Master Fund, Ltd. &quot;FEVIBM&quot; as well as to 21 April LP and 21 April Ltd. (collectively, the &quot;Funds&quot;) and (iii) a subsidiary of First Eagle Holdings, Inc. (&quot;FEHI&quot;). FEHI disclaims Section 16 beneficial ownership of the shares held directly by the Funds except to the extent, if any, of its pecuniary interest therein, and this report shall not be deemed an admission that FEHI is the Section 16 beneficial owner of any such securities.</footnote>
        <footnote id="F2">Represents 3,322,406, 12,953,420, and 5,586,944 shares held directly by 21 April LP, 21 April Ltd. and FEVIBM, respectively. FEHI disclaims Section 16 beneficial ownership of the shares held directly by the Funds except to the extent, if any, of its pecuniary interest therein, and this report shall not be deemed an admission that FEHI is the Section 16 beneficial owner of any such securities.</footnote>
        <footnote id="F3">Represents 63,678 and 255,098 shares purchased by 21 April LP and 21 April Ltd., respectively. FEHI disclaims Section 16 beneficial ownership of the shares held directly by the Funds except to the extent, if any, of its pecuniary interest therein, and this report shall not be deemed an admission that FEHI is the Section 16 beneficial owner of any such securities.</footnote>
        <footnote id="F4">Represents 3,386,084, 13,208,518, and 5,586,944 shares held directly by 21 April LP, 21 April Ltd. and FEVIBM, respectively. FEHI disclaims Section 16 beneficial ownership of the shares held directly by the Funds except to the extent, if any, of its pecuniary interest therein, and this report shall not be deemed an admission that FEHI is the Section 16 beneficial owner of any such securities.</footnote>
        <footnote id="F5">Represents 40,809 and 159,191 shares purchased by 21 April LP and 21 April Ltd., respectively. FEHI disclaims Section 16 beneficial ownership of the shares held directly by the Funds except to the extent, if any, of its pecuniary interest therein, and this report shall not be deemed an admission that FEHI is the Section 16 beneficial owner of any such securities.</footnote>
        <footnote id="F6">Represents 3,426,893, 13,367,709, and 5,586,944 shares held directly by 21 April LP, 21 April Ltd. and FEVIBM, respectively. FEHI disclaims Section 16 beneficial ownership of the shares held directly by the Funds except to the extent, if any, of its pecuniary interest therein, and this report shall not be deemed an admission that FEHI is the Section 16 beneficial owner of any such securities.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>FIRST EAGLE INVESTMENT MANAGEMENT, LLC, as General Partner of 21 April LP and Investment Adviser of 21 April Ltd. and FEVIBM, By: /s/ Michael M. Kellen, Director</signatureName>
        <signatureDate>2016-09-28</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>FIRST EAGLE HOLDINGS, INC., By: /s/ Michael M. Kellen, Director</signatureName>
        <signatureDate>2016-09-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
