Antitussive activity of diphenhydramine in chronic cough

A double‐blind crossover study was conducted to evaluate the antitussive effectiveness of diphenhydramine (DPH) in chronic coughs related to bronchitis, at doses of25 and 50 mg every 4 hr for four doses. Both 25− and 50‐mg doses caused a statistically and clinically significant reduction in frequency of coughs, compared to placebo. The most frequently reported side effect was drowsiness, principally at the 50‐mg dose level. There was little or no apparent correlation between antitussive effectiveness and incidence of drowsiness, however, suggesting that the two effects were pharmacologically unrelated.