ABSTRACTBackgroundN‐acetylcysteine is used as prophylaxis to prevent contrast‐induced nephropathy.AimTo investigate the stability of a formulated N‐acetylcysteine capsule for prevention of contrast‐induced nephropathy.MethodGelatine capsules containing N‐acetylcysteine and lactose were stored at 2–8 °C, 18–25 °C and 40 °C, with and without desiccant, at ambient and high humidity (~75%). Capsule stability was assessed using high performance liquid chromatography to measure N‐acetylcysteine and N,N‐diacetylcystine (breakdown product) on Days 0, 3, 7, 14, 28, 84, 168, 252, 336 and 420 of storage.ResultsAt 40 °C and high humidity the capsules showed severe physical degradation and hydrogen sulphide odour, and loss of N‐acetylcysteine was around 30–40% by 420 days. Under all other storage conditions, the N‐acetylcysteine content was stable up to 420 days (± 2% change from initial content), irrespective of humidity. The presence of desiccant did not significantly affect stability and the presence of N,N‐diacetylcystine barely exceeded the European Pharmacopoeia limit of 0.5%.ConclusionThe formulated N‐acetylcysteine capsule was stable for 420 days when stored at 2–8 °C, 18–25 °C and 40 °C.