Organization and operations	12 Months Ended
Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and operations	Organization and operations The Company Flex Pharma, Inc. (the "Company") is a biotechnology company that is developing innovative and proprietary treatments for muscle cramps and spasms associated with severe neurological conditions and exercise-associated muscle cramps. The Company's lead drug product candidate, FLX-787, is currently in exploratory Phase 2 clinical trials in Australia in patients with multiple sclerosis, or MS, and amyotrophic lateral sclerosis, or ALS. In 2017, the Company expects to initiate Phase 2 clinical trials in the United States of FLX-787 in patients with motor neuron disease, primarily with ALS, and patients with Charcot-Marie-Tooth disease, or CMT. In 2016, the Company launched its consumer product, HOTSHOT®, to prevent and treat exercise-associated muscle cramps, or EAMCs. FLX-787, HOTSHOT and the Company's other product candidates are based on the potential mechanism of action we describe as Chemical Neuro Stimulation, which is the process by which a chemical signal, acting topically, induces a neuronal sensory signal that produces a beneficial effect. The Company's product candidates activate certain receptors in primary sensory neurons, which then act via neuronal circuits to reduce the repetitive firing, or hyperexcitability, of alpha-motor neurons in the spinal cord, thereby preventing or reducing the frequency and intensity of muscle cramps and spasms. In connection with the launch of HOTSHOT, the Company began operating as two reportable segments, Consumer Operations and Drug Development. See Note 15 for additional discussion and information on the Company's reportable segments. The Company is subject to risks common to companies in the biotechnology and consumer products industries, including, but not limited to, risks of failure of pre-clinical studies, clinical studies and clinical trials, the need to obtain marketing approval for its drug product candidates, the need to successfully commercialize and gain market acceptance of its drug product candidates and its consumer products, dependence on key personnel, protection of proprietary technology, compliance with government regulations and development by competitors of alternative products. In February 2015, the Company sold 5,491,191 shares of common stock (inclusive of 91,191 shares of common stock sold by the Company pursuant to the exercise of an overallotment option granted to the underwriters in connection with the offering) through an underwritten initial public offering ("IPO") at a price of $16.00 per share. The aggregate net proceeds received by the Company from the offering were approximately $79,900,000, after deducting underwriting discounts and commissions and offering expenses payable by the Company of approximately $8,000,000 (See Note 2). Liquidity The Company has incurred an accumulated deficit of $76,645,613 from February 26, 2014 (inception) through December 31, 2016, and will require substantial additional capital to fund its research and development and the growth of its consumer brand and HOTSHOT. The Company had cash, cash equivalents and marketable securities of $61,074,973 at December 31, 2016. The Company believes that its existing cash, cash equivalents and marketable securities will be sufficient to allow the Company to fund its current operating plan for at least 12 months from the date the financial statements are issued. Management expects the Company to incur a loss for the foreseeable future. The Company's ability to achieve profitability in the future is dependent upon the successful development, approval and commercialization of its drug product candidates and successful commercialization of HOTSHOT and future consumer products, and achieving a level of revenues adequate to support the Company's cost structure. The Company may never achieve profitability, and unless and until it does, the Company will continue to need to raise additional capital. Management intends to fund future operations through additional private or public debt or equity offerings, and may seek additional capital through arrangements with collaborators or from other sources. There can be no assurances, however, that additional funding will be available on terms acceptable to the Company, or at all.

Organization and operations

The Company

Flex Pharma, Inc. (the "Company") is a biotechnology company that is developing innovative and proprietary treatments for muscle cramps and spasms associated with severe neurological conditions and exercise-associated muscle cramps. The Company's lead drug product candidate, FLX-787, is currently in exploratory Phase 2 clinical trials in Australia in patients with multiple sclerosis, or MS, and amyotrophic lateral sclerosis, or ALS. In 2017, the Company expects to initiate Phase 2 clinical trials in the United States of FLX-787 in patients with motor neuron disease, primarily with ALS, and patients with Charcot-Marie-Tooth disease, or CMT. In 2016, the Company launched its consumer product, HOTSHOT®, to prevent and treat exercise-associated muscle cramps, or EAMCs.

FLX-787, HOTSHOT and the Company's other product candidates are based on the potential mechanism of action we describe as Chemical Neuro Stimulation, which is the process by which a chemical signal, acting topically, induces a neuronal sensory signal that produces a beneficial effect. The Company's product candidates activate certain receptors in primary sensory neurons, which then act via neuronal circuits to reduce the repetitive firing, or hyperexcitability, of alpha-motor neurons in the spinal cord, thereby preventing or reducing the frequency and intensity of muscle cramps and spasms.

In connection with the launch of HOTSHOT, the Company began operating as two reportable segments, Consumer Operations and Drug Development. See Note 15 for additional discussion and information on the Company's reportable segments.

The Company is subject to risks common to companies in the biotechnology and consumer products industries, including, but not limited to, risks of failure of pre-clinical studies, clinical studies and clinical trials, the need to obtain marketing approval for its drug product candidates, the need to successfully commercialize and gain market acceptance of its drug product candidates and its consumer products, dependence on key personnel, protection of proprietary technology, compliance with government regulations and development by competitors of alternative products.

In February 2015, the Company sold 5,491,191 shares of common stock (inclusive of 91,191 shares of common stock sold by the Company pursuant to the exercise of an overallotment option granted to the underwriters in connection with the offering) through an underwritten initial public offering ("IPO") at a price of $16.00 per share. The aggregate net proceeds received by the Company from the offering were approximately $79,900,000, after deducting underwriting discounts and commissions and offering expenses payable by the Company of approximately $8,000,000 (See Note 2).

Liquidity

The Company has incurred an accumulated deficit of $76,645,613 from February 26, 2014 (inception) through December 31, 2016, and will require substantial additional capital to fund its research and development and the growth of its consumer brand and HOTSHOT. The Company had cash, cash equivalents and marketable securities of $61,074,973 at December 31, 2016. The Company believes that its existing cash, cash equivalents and marketable securities will be sufficient to allow the Company to fund its current operating plan for at least 12 months from the date the financial statements are issued. Management expects the Company to incur a loss for the foreseeable future. The Company's ability to achieve profitability in the future is dependent upon the successful development, approval and commercialization of its drug product candidates and successful commercialization of HOTSHOT and future consumer products, and achieving a level of revenues adequate to support the Company's cost structure. The Company may never achieve profitability, and unless and until it does, the Company will continue to need to raise additional capital. Management intends to fund future operations through additional private or public debt or equity offerings, and may seek additional capital through arrangements with collaborators or from other sources. There can be no assurances, however, that additional funding will be available on terms acceptable to the Company, or at all.