Document And Entity Information [Abstract] Document and Entity Information [Abstract] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus (Q1,Q2,Q3,FY) Document Fiscal Period Focus Amendment Flag Amendment Flag Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Organization, Consolidation and Presentation of Financial Statements [Abstract] Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Subsidiary or Equity Method Investee, Sale of Stock by Subsidiary or Equity Investee [Table] Sale of Stock [Axis] Sale of Stock [Axis] Sale of Stock [Domain] Sale of Stock [Domain] IPO [Member] IPO [Member] Over-Allotment Option [Member] Over-Allotment Option [Member] Subsidiary, Sale of Stock [Line Items] Subsidiary, Sale of Stock [Line Items] Common stock, shares issued Stock Issued During Period, Shares, New Issues Common stock, price per share (in usd per share) Shares Issued, Price Per Share Proceeds from initial public offering, net of offering costs Proceeds from Issuance Initial Public Offering Underwriting discounts, commissions, and offering expenses Payments of Stock Issuance Costs Retained earnings (accumulated deficit) Retained Earnings (Accumulated Deficit) Cash Cash Statement of Cash Flows [Abstract] Statement [Table] Statement [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Series A preferred stock Series A Preferred Stock [Member] Series B Preferred Stock Series B Preferred Stock [Member] Statement [Line Items] Statement [Line Items] Operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Net loss Net Income (Loss) Attributable to Parent Adjustments to reconcile net loss to net cash (used in) provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation Stock-based compensation Share-based Compensation Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Restricted cash Increase (Decrease) in Restricted Cash for Operating Activities Prepaid expenses and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Other assets Increase (Decrease) in Other Noncurrent Assets Accounts payable Increase (Decrease) in Accounts Payable Accrued expenses Increase (Decrease) in Accrued Liabilities Deferred rent Increase (Decrease) in Deferred Liabilities Net cash (used in) provided by operating activities Net Cash Provided by (Used in) Operating Activities Investing activities Net Cash Provided by (Used in) Investing Activities [Abstract] Purchases of property and equipment Payments to Acquire Property, Plant, and Equipment Net cash used in by investing activities Net Cash Provided by (Used in) Investing Activities Financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from issuance of convertible preferred stock, net of issuance costs Proceeds from Issuance of Convertible Preferred Stock Proceeds from sale of restricted common stock to founders Proceeds From Sale Of Restricted Common Stock Proceeds From Sale Of Restricted Common Stock Proceeds from early exercise of stock options Proceeds from Stock Options Exercised Net cash provided by financing activities Net Cash Provided by (Used in) Financing Activities Net increase in cash Cash, Period Increase (Decrease) Cash at beginning of period Cash at end of period Supplemental cash flow information Supplemental Cash Flow Information [Abstract] Property and equipment purchases included in accounts payable Property And Equipment Purchases In Accrued Expense Property and Equipment Purchases In Accrued Expense IPO issuance costs included in accounts payable and accrued expenses at December 31, 2014 Deferred Initial Public Offering Issuance Costs In Accounts Payable and Accrued Expenses Deferred Initial Public Offering Issuance Costs In Accounts Payable and Accrued Expenses IPO issuance costs paid in cash through December 31, 2014 Initial Public Offering Costs Paid Initial Public Offering Costs Paid Disclosure of Compensation Related Costs, Share-based Payments [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Flex Pharma, Inc. 2014 Equity Incentive Plan Flex Pharma, Inc. 2014 Equity Incentive Plan [Member] Flex Pharma, Inc. 2014 Equity Incentive Plan [Member] 2015 Equity Incentive Plan Equity Incentive Plan 2015 [Member] Equity Incentive Plan 2015 [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Shares authorized Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Shares remaining available for grant of stock awards Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Stock options, vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Stock options, contractual term Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period Unrecognized compensation cost Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options Unrecognized compensation cost, recognition period Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Statement of Financial Position [Abstract] Preferred stock, par value (in usd per share) Preferred Stock, Par or Stated Value Per Share Preferred stock, shares authorized Preferred Stock, Shares Authorized Preferred stock, shares issued Preferred Stock, Shares Issued Preferred stock, shares outstanding Preferred Stock, Shares Outstanding Common stock, par value (in usd per share) Common Stock, Par or Stated Value Per Share Common stock, shares authorized Common Stock, Shares Authorized Common stock, shares issued Common Stock, Shares, Issued Common stock, shares outstanding Common Stock, Shares, Outstanding Earnings Per Share [Abstract] Schedule of antidilutive securities excluded from computation of earnings per share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Equity [Abstract] Restricted common stock activity Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Unvested restricted common stock sold to founders Restricted Stock [Member] Unvested restricted common stock issued upon early exercise of stock options Nonvested Restricted Common Stock Issued Upon Early Exercise Of Options [Member] Nonvested Restricted Common Stock Issued Upon Early Exercise Of Options [Member] Options to purchase common stock Employee Stock Option [Member] Preferred stock Preferred Stock [Member] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive securities excluded from computation of earnings per share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Organization and operations Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Net loss per share Earnings Per Share [Text Block] Payables and Accruals [Abstract] Accrued expenses and other current liabilities Accounts Payable, Accrued Liabilities, and Other Liabilities Disclosure, Current [Text Block] Accounting Policies [Abstract] Summary of significant accounting policies and recent accounting pronouncements Significant Accounting Policies [Text Block] Vesting [Axis] Vesting [Axis] Vesting [Domain] Vesting [Domain] Vests upon issuance Share-based Compensation Award, Tranche One [Member] Vests ratably over four years Share-based Compensation Award, Tranche Two [Member] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Employee stock option Restricted common stock, shares sold Restricted common stock, price per share (in usd per share) Sale of Stock, Price Per Share Restricted common stock, percent vested Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage Restricted common stock, vesting period Restricted common stock, repurchase period Stock Repurchase Program, Period in Force Restricted common stock, repurchase price (in usd per share) Treasury Stock Acquired, Average Cost Per Share Restricted common stock, shares outstanding Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number Shares exercised Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Shares exercised, weighted-average exercise price (in usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price Income Tax Disclosure [Abstract] Income taxes Income Tax Disclosure [Text Block] Basis of presentation Basis of Accounting, Policy [Policy Text Block] Use of estimates Use of Estimates, Policy [Policy Text Block] Principles of consolidation Consolidation, Policy [Policy Text Block] Recent accounting pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Summary of stock option activity Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block] Summary of stock-based compensation expense Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] Common stock Stockholders' Equity Note Disclosure [Text Block] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Research and development Research and Development Expense [Member] General and administrative General and Administrative Expense [Member] Employee Service Share-Based Compensation Allocation of Recognized Period Costs [Line Items] Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items] Stock-based compensation expense Allocated Share-based Compensation Expense Common stock, number of shares issued from conversion of preferred stock (in shares) Conversion of Stock, Shares Converted IPO issuance cost Deferred Offering Costs Reverse stock split conversion ratio Stockholders' Equity Note, Stock Split, Conversion Ratio Income Statement [Abstract] Operating expenses: Operating Expenses [Abstract] Research and development Research and Development Expense General and administrative General and Administrative Expense Total operating expenses Operating Expenses Loss from operations Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest Interest income Interest Income, Operating Net loss Comprehensive loss Comprehensive Income (Loss), Net of Tax, Attributable to Parent Net loss per share attributable to common stockholders - basic and diluted (in usd per share) Earnings Per Share, Basic and Diluted Weighted-average number of common shares outstanding — basic and diluted Weighted Average Number of Shares Outstanding, Basic and Diluted Series A convertible preferred stock Assets Assets [Abstract] Current assets: Assets, Current [Abstract] Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Total current assets Assets, Current Property and equipment, net Property, Plant and Equipment, Net Deferred IPO issuance costs Deferred Finance Costs, Noncurrent, Net Deferred tax assets Deferred Tax Assets, Net of Valuation Allowance, Noncurrent Other assets Other Assets, Noncurrent Restricted cash Restricted Cash and Cash Equivalents, Noncurrent Total assets Assets Liabilities, convertible preferred stock and stockholders' equity (deficit) Liabilities and Equity [Abstract] Current liabilities: Liabilities, Current [Abstract] Accounts payable Accounts Payable, Current Accrued expenses and other current liabilities Accrued Expenses And Other Current Liabilities Accrued Expenses And Other Current Liabilities Deferred tax liabilities Deferred Tax Liabilities, Net, Current Deferred rent, current portion Deferred Rent Credit, Current Total current liabilities Liabilities, Current Deferred rent, net of current portion Deferred Rent Credit, Noncurrent Other long term liabilities Other Liabilities, Noncurrent Total liabilities Liabilities Convertible preferred stock: Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Preferred stock, carrying value Preferred Stock, Value, Issued Stockholders' equity (deficit): Stockholders' Equity Attributable to Parent [Abstract] Common stock, $0.0001 par value; 100,000,000 and 61,000,000 shares authorized at March 31, 2015 and December 31, 2014, respectively, 17,933,664 and 5,434,301 shares issued at March 31, 2015 and December 31, 2014, respectively, and 14,932,098 and 2,215,711 shares outstanding at March 31, 2015 and December 31, 2014, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Accumulated deficit Total stockholders' equity (deficit) Stockholders' Equity Attributable to Parent Total liabilities, convertible preferred stock and stockholders' equity (deficit) Liabilities and Equity Stock-based compensation Disclosure of Compensation Related Costs, Share-based Payments [Text Block] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward] Shares outstanding, Beginning Balance Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number Shares granted Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Shares exercised Shares canceled or forfeited Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures in Period Shares outstanding, Ending Balance Shares exercisable Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Shares vested or expected to vest Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract] Shares outstanding, weighted-average exercise price, Ending Balance (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price Shares granted, weighted-average exercise price (in usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price Shares canceled or forfeited, weighted-average exercise price (in usd per share) Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price Shares outstanding, weighted-average exercise price, Beginning Balance (in usd per share) Shares exercisable, weighted-average exercise price (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price Shares vested or expected to vest, weighted-average exercise price (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract] Outstanding, weighted-average remaining contractual term Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term Exercisable, weighted-average remaining contractual term Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Remaining Contractual Term Vested or expected to vest, weighted-average remaining contractual term Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Remaining Contractual Term Outstanding, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value Exercisable, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value Vested or expected to vest, aggregate intrinsic value Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Aggregate Intrinsic Value Payroll and employee-related costs Employee-related Liabilities, Current Consumer brand-related costs Accrued Marketing Costs, Current Research and development costs Accrued Research And Development Expense, Current Accrued Research and Development Expense, Current Professional fees Accrued Professional Fees, Current Other Other Accrued Liabilities, Current Deferred IPO issuance costs Deferred IPO Issuance Cost, Current Deferred IPO Issuance Cost, Current Total Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Beginning balance, non-vested, number of shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Vested, number of shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Ending balance, non-vested, number of shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Beginning balance, weighted average grant date fair value (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Vested, weighted average grant date fair value (in usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Ending balance, weighted average grant date fair value (in usd per share) Schedule of accrued expenses Schedule of Accrued Liabilities [Table Text Block] Schedule of other current liabilities Other Current Liabilities [Table Text Block]