Organization and operations	3 Months Ended
Organization and operations	Mar. 31, 2017
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 the Company describes 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. The Company operates as two reportable segments, Consumer Operations and Drug Development. See Note 11 for additional discussion and information on the 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 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. Liquidity The Company has incurred an accumulated deficit of $84,914,008 since inception and will require substantial additional capital to fund its research and development and commercialization and growth of its consumer brand and HOTSHOT. The Company had unrestricted cash, cash equivalents and marketable securities of $52,814,770 at March 31, 2017. 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.