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Stock-based compensation - Narrative (Details) (USD $)
10 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Apr. 30, 2014
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock-based compensation expense $ 1,462,061us-gaap_AllocatedShareBasedCompensationExpense      
Shares granted 1,020,234us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross      
Shares granted, weighted-average grant date fair value (in usd per share) $ 2.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue      
Shares exercised 13,572us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised      
Shares exercised, weighted average exercise price (in usd per share) $ 0.60us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice      
Stock options exercised, intrinsic value 51,719us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue      
Unrecognized compensation cost 12,632,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions      
Unrecognized compensation cost, recognition period 3 years 3 months 7 days      
Non-employee Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares granted to non-employee consultants and members of the Scientific Advisory Board 137,765flks_SharebasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodNonEmployee
/ us-gaap_AwardTypeAxis
= flks_NonemployeeStockOptionsMember
     
Stock options, vesting period 4 years      
Stock options, contractual term 10 years      
Stock-based compensation expense $ 149,000us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= flks_NonemployeeStockOptionsMember
     
Flex Pharma, Inc. 2014 Equity Incentive Plan        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares authorized   2,070,200us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= flks_FlexPharmaInc.2014EquityIncentivePlanMember
1,451,087us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= flks_FlexPharmaInc.2014EquityIncentivePlanMember
116,754us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= flks_FlexPharmaInc.2014EquityIncentivePlanMember
Shares remaining available for grant of stock awards 262,482us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= flks_FlexPharmaInc.2014EquityIncentivePlanMember