April 30, 2019 William McVicar, Ph.D. President and Chief Executive Officer Flex Pharma, Inc. 31 St. James Avenue, 6th Floor Boston, MA 02116 Re: Flex Pharma, Inc. Registration Statement on Form S-3 Filed April 24, 2019 File No. 333-231010 Dear Dr. McVicar: This is to advise you that we have not reviewed and will not review your registration statement. Please refer to Rules 460 and 461 regarding requests for acceleration. We remind you that the company and its management are responsible for the accuracy and adequacy of their disclosures, notwithstanding any review, comments, action or absence of action by the staff. Please contact Jeffrey Gabor at 202-551-2544 with any questions. Sincerely, Division of Corporation Finance Office of Healthcare & Insurance cc: Jaime L. Chase, Esq.