<SEC-DOCUMENT>0001104659-18-062810.txt : 20181018
<SEC-HEADER>0001104659-18-062810.hdr.sgml : 20181018
<ACCEPTANCE-DATETIME>20181018170341
ACCESSION NUMBER:		0001104659-18-062810
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20181016
FILED AS OF DATE:		20181018
DATE AS OF CHANGE:		20181018

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Longwood Fund II, L.P.
		CENTRAL INDEX KEY:			0001543245
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-36812
		FILM NUMBER:		181128831

	BUSINESS ADDRESS:	
		STREET 1:		800 BOYLSTON STREET
		STREET 2:		SUITE 1555
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02199
		BUSINESS PHONE:		617-351-2590

	MAIL ADDRESS:	
		STREET 1:		800 BOYLSTON STREET
		STREET 2:		SUITE 1555
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02199

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Flex Pharma, Inc.
		CENTRAL INDEX KEY:			0001615219
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				465087339
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		800 BOYLSTON STREET
		STREET 2:		24TH FLOOR
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02199
		BUSINESS PHONE:		617-874-1821

	MAIL ADDRESS:	
		STREET 1:		800 BOYLSTON STREET
		STREET 2:		24TH FLOOR
		CITY:			BOSTON
		STATE:			MA
		ZIP:			02199
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>a4.xml
<DESCRIPTION>4
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2018-10-16</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001615219</issuerCik>
        <issuerName>Flex Pharma, Inc.</issuerName>
        <issuerTradingSymbol>FLKS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001543245</rptOwnerCik>
            <rptOwnerName>Longwood Fund II, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O FLEX PHARMA, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>800 BOYLSTON STREET</rptOwnerStreet2>
            <rptOwnerCity>BOSTON</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02199</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2018-10-16</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>3212861</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.9154</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $0.70 to $1.33, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (1) to this Form 4.</footnote>
        <footnote id="F2">Represents 2,697,264 shares sold by Longwood Fund II, L.P. (&quot;Longwood Fund II&quot;) and 515,597 shares sold by Longwood Fund III, L.P. (&quot;Longwood Fund III&quot;).</footnote>
        <footnote id="F3">Longwood Fund II GP, LLC is the General Partner of Longwood Fund II and Longwood Fund III GP, LLC is the General Partner of Longwood Fund III. Christoph Westphal, M.D., Ph.D. and Richard Aldrich (collectively, the &quot;Managers&quot;) are managers of Longwood Fund II GP, LLC and Longwood Fund III GP, LLC. The Managers have delegated their voting and dispositive power with respect to the Issuer's securities held by Longwood Fund II and Longwood Fund III (the &quot;Longwood Shares&quot;) to a committee comprised of three members, two of which such members include the Managers (the &quot;Longwood Committee&quot;).</footnote>
        <footnote id="F4">The Longwood Committee approves investment and voting decisions by a majority vote, and no individual member has sole control or voting power over the Longwood Shares.  Each of Longwood Fund II GP, LLC, Longwood Fund III GP, LLC, the Managers and the members of the Longwood Committee disclaim beneficial ownership of the Longwood Shares, except to the extent of their respective pecuniary interest therein, and the inclusion of these shares in this report shall not be deemed an admission of beneficial ownership of the shares for purposes of Section 16 or for any other purpose.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ John Lawrence, Attorney-in-Fact</signatureName>
        <signatureDate>2018-10-18</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
