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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-01-01</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000740260</issuerCik>
        <issuerName>VENTAS INC</issuerName>
        <issuerTradingSymbol>VTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001198222</rptOwnerCik>
            <rptOwnerName>ROSENBERG SHELI  Z</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>155 NORTH WACKER DRIVE</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-01-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2269</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>84978.672</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Restricted stock granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan. This restricted stock vests in two equal annual installments beginning on the first anniversary of the date of the grant.</footnote>
        <footnote id="F2">Includes 522.639 shares acquired between September 12, 2013 and January 2, 2014 under the Ventas Employee and Director Stock Purchase Plan.</footnote>
        <footnote id="F3">Includes 1,128.309 shares acquired between September 27, 2013 and December 31, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.</footnote>
        <footnote id="F4">As of January 1, 2014, Reporting Person also owns options to purchase 40,000 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2014-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2014-01-01
0
0000740260
VENTAS INC
VTR
0001198222
ROSENBERG SHELI  Z
155 NORTH WACKER DRIVE
CHICAGO
IL
60606
1
0
0
0
Common Stock
2014-01-01
4
A
0
2269
0
A
84978.672
D
Restricted stock granted by Issuer pursuant to the Ventas, Inc. 2012 Incentive Plan. This restricted stock vests in two equal annual installments beginning on the first anniversary of the date of the grant.
Includes 522.639 shares acquired between September 12, 2013 and January 2, 2014 under the Ventas Employee and Director Stock Purchase Plan.
Includes 1,128.309 shares acquired between September 27, 2013 and December 31, 2013 under the Ventas, Inc. Distribution Reinvestment and Stock Purchase Plan.
As of January 1, 2014, Reporting Person also owns options to purchase 40,000 shares of Issuer's common stock.
Sheli Z. Rosenberg, By: T. Richard Riney, Attorney-In-Fact
2014-01-03