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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2003-10-07</periodOfReport>

    <dateOfOriginalSubmission>2003-10-28</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000003116</issuerCik>
        <issuerName>AKORN INC</issuerName>
        <issuerTradingSymbol>AKN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001033150</rptOwnerCik>
            <rptOwnerName>KAPOOR JOHN N</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>225 EAST DEERPATH ROAD</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 250</rptOwnerStreet2>
            <rptOwnerCity>LAKE FOREST</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60045</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Series A 6% Participating Conv.Pfd.Stock (right to convert)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0.75</value>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2003-10-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>107350</value>
                    <footnoteId id="F5"/>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2003-10-07</value>
            </exerciseDate>
            <expirationDate>
                <value>2011-10-31</value>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>14313333</value>
                    <footnoteId id="F4"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>107350</value>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Common Stock of Akorn, Inc. (&quot;Shares&quot;) and securities convertible into such Shares are held through various limited partnerships and trusts.  Dr. Kapoor disclaims beneficial ownership of such shares except to the extent of his pecuniary interest.</footnote>
        <footnote id="F2">Subject to adjustment.</footnote>
        <footnote id="F3">Mandatory redemption on this date.</footnote>
        <footnote id="F4">On October 7, 2003 the Trust, along with other investors, consummated a recapitalization transaction involving Akorn, Inc. by purchasing all of Akorn, Inc.'s outstanding senior debt from The Northern Trust Company.  In return for its contribution, the Trust received from Akorn: 107,350 shares of Series A 6% Participating Convertible Preferred Stock, which are convertible as of the date of issuance into 14,313,333 Shares, and warrants to purchase 3,578,333 Shares at $1.00 per share.  Additionally, the Trust made a loan to Akorn in the principal amount of $2,117,139.03.  In consideration for its loan, the Trust received warrants to purchase 211,714 Shares at $1.10 per share.  Separately, the Trust agreed to serve as guarantor for a credit facility between LaSalle National Bank Association, Akorn, Inc., and Akorn (New Jersey), Inc., for which it received warrants to purchase 880,000 Shares at $1.10 per share.</footnote>
        <footnote id="F5">This amendment is being filed to report that accrued dividends will be converted into a right to receive common stock at $0.75 per share at the end of each quarter if the dividends are not paid in cash, and that each such accrual will therefore increase the number of shares that may be acquired or disposed of under the reported derivative security.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ John N. Kapoor</signatureName>
        <signatureDate>2005-05-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
