Antitussive activity of diphenhydramine in chronic cough.

A double-blind crossover study was conducted to evaluate the antitussive effectiveness of diphenhydramine (DPH) in chornic coughs related to bronchitis, at doses of 25 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.