<?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>0000939563</rptOwnerCik>
            <rptOwnerName>GEARY RONALD G</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>2700 GAST BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LOUISVILLE</rptOwnerCity>
            <rptOwnerState>KY</rptOwnerState>
            <rptOwnerZipCode>40205</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>21619.697</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </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 2,641 additional shares of common stock to reflect shares that were held in street name in the Reporting Person's brokerage account prior to May 15, 2009.  Such shares were inadvertently omitted from the Reporting Person's reported holdings prior to May 15, 2009 and were reported as disposed of on the Reporting Person's Form 4 filed on May 19, 2009.</footnote>
        <footnote id="F3">As of January 1, 2014, Reporting Person also owns options to purchase 2,500 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Ronald G. Geary, 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
0000939563
GEARY RONALD G
2700 GAST BLVD.
LOUISVILLE
KY
40205
1
0
0
0
Common Stock
2014-01-01
4
A
0
2269
0
A
21619.697
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 2,641 additional shares of common stock to reflect shares that were held in street name in the Reporting Person's brokerage account prior to May 15, 2009.  Such shares were inadvertently omitted from the Reporting Person's reported holdings prior to May 15, 2009 and were reported as disposed of on the Reporting Person's Form 4 filed on May 19, 2009.
As of January 1, 2014, Reporting Person also owns options to purchase 2,500 shares of Issuer's common stock.
Ronald G. Geary, By: T. Richard Riney, Attorney-In-Fact
2014-01-03