As filed with the Securities and Exchange Commission on January 28, 2015

Registration No. 333-          

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Flex Pharma, Inc.

(Exact Name of Registrant as Specified in Its Charter)

800 Boylston Street, 24th Floor

Boston, MA 02199

(617) 874-1821

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Christoph Westphal, M.D., Ph.D.

President and Chief Executive Officer

Flex Pharma, Inc.

800 Boylston Street, 24th Floor

Boston, MA 02199

(617) 874-1821

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service)

Copies to:

Approximate date of commencement of proposed sale to the public: As soon as practicable after this registration statement becomes effective.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, as amended (the “Securities Act”), check the following box.  o

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  x (File No. 333-201276)

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  o

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

CALCULATION OF REGISTRATION FEE

(1)          The shares being registered pursuant to this Registration Statement are in addition to the 5,307,250 shares registered pursuant to the Registrant’s Registration Statement on Form S-1 (Registration No. 333-201276). Includes 117,750 shares that the underwriters have the option to purchase.

(2)   Based on the initial public offering price.

(3)   Calculated pursuant to Rule 457(a) under the Securities Act of 1933, as amended.

This Registration Statement shall become effective upon filing in accordance with Rule 462(b) under the Securities Act of 1933.

EXPLANATORY NOTE AND

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

This Registration Statement is being filed with the Securities and Exchange Commission (the “Commission”) with respect to the registration of additional shares of common stock, par value $0.0001 per share, of Flex Pharma, Inc., a Delaware corporation,  pursuant to Rule 462(b) under the Securities Act of 1933, as amended. This Registration Statement incorporates by reference the contents of, including all amendments and exhibits thereto and all information incorporated by reference therein, the Registration Statement on Form S-1 (Registration No. 333-201276), which was declared effective by the Commission on January 28, 2015, and is being filed solely for the purpose of increasing the aggregate number of shares to be offered in the public offering by 902,750 shares including the shares that may be sold pursuant to the underwriters’ option to purchase additional shares.

The required opinions and consents are listed on the Exhibit Index attached hereto.

SIGNATURES

Pursuant to the requirements of the Securities Act, the Registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, State of Massachusetts, on the 28th day of January, 2015.

Pursuant to the requirements of the Securities Act, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

*Pursuant to power of attorney

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EXHIBIT INDEX

(1)                                 Included on the signature page of Registration Statement on Form S-1 (File No. 333-201276), filed with the Securities and Exchange Commission on January 13, 2015, and incorporated herein by reference.

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Exhibit 5.1

Marc A. Recht

T: +1 617 937 2316

mrecht@cooley.com

January 28, 2015

Flex Pharma, Inc.

800 Boylston Street, 24th Floor

Boston, Massachusetts 02199

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the filing by Flex Pharma, Inc., a Delaware corporation (the “Company “), of a registration statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission pursuant to Rule 462(b) promulgated under the Securities Act of 1933, as amended, covering an underwritten public offering of up to 902,750 shares of the Company’s common stock, par value $0.0001 per share (the “Shares”), to be sold by the Company. The Registration Statement incorporates by reference the registration statement on Form S-1 (No. 333-201276), which was declared effective on January 28, 2015 (the “Prior Registration Statement”), including the prospectus which forms a part of the Prior Registration Statement (the “Prospectus”).  We are acting as counsel for the Company.

In connection with this opinion, we have examined and relied upon (a) the Registration Statement, the Prior Registration Statement and the Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation, as amended, and Bylaws, as currently in effect, (c) the Company’s Amended and Restated Certificate of Incorporation, filed as Exhibit 3.5 to the Prior Registration Statement and the Company’s Bylaws, filed as Exhibit 3.7 to the Prior Registration Statement, each of which is to be in effect immediately after the closing of the offering contemplated by the Registration Statement, and (d) the originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below.  We have assumed the genuineness and authenticity of all documents submitted to us as originals and the conformity to originals of all documents submitted to us as copies.  As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not sought independently to verify such matters.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware.  We express no opinion as to whether the laws of any particular jurisdiction are applicable to the subject matter hereof.  We are not rendering any opinion as to compliance with any federal or state antifraud law, rule or regulation relating to securities, or to the sale or issuance thereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued against payment therefor as described in the Registration Statement and the Prospectus, will be validly issued, fully paid and non-assessable.

500 BOYLSTON STREET, BOSTON, MA 02116-3736  T: (617) 937-2300  F: (617) 937-2400  WWW.COOLEY.COM

We consent to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

Cooley LLP

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Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

We consent to the incorporation by reference in the Registration Statement on Form S-1 filed pursuant to Rule 462(b) of the Securities Act of 1933 of the reference to our firm under the caption “Experts” and to the incorporation by reference of our report dated December 8, 2014 (except for Note 13(D)(i), as to which the date is January 20, 2015) relating to the financial statements of Flex Pharma, Inc. included in Amendment No. 2 to the Registration Statement (Form S-1 No. 333-201276) and related Prospectus of Flex Pharma, Inc. for the registration of its common stock.