Late-breaking abstract: Antitussive effect of a diphenhydramine-containing multicomponent cough syrup in acute viral cough

Background Cough is among the most common complaints for which patients seek medical attention. Currently available over-the-counter cough remedies historically have been criticized for lack of scientific evidence supporting their efficacy. Although the first-generation antihistamine diphenhydramine is classified as an antitussive by the US Food and Drug Administration (Fed Regist 1994;59:29172-4), to the authors9 knowledge it has never been shown to inhibit cough reflex sensitivity in subjects with pathological cough. Methods 22 subjects with acute viral respiratory tract infection (common cold) underwent cough reflex sensitivity measurement employing capsaicin challenge on 3 separate days, 2 hours after ingesting a single dose of study drug (to coincide with peak blood levels) that was administered in a randomized, double-blind manner: a multicomponent syrup containing diphenhydramine (25 mg), phenylephrine (10 mg), in a natural cocoa formulation (Dr. Cocoa, Infirst Healthcare, UK); dextromethorphan (30 mg) syrup; and, placebo syrup. The standard endpoint of cough challenge was used: the concentration of capsaicin inducing ≥5 coughs (C5). Results Differences in log C5 responses were analyzed by 1-way ANOVA for dependent samples combined with Tukey9s HSD test for pairwise comparisons. A significant difference (p=0.0024) was established among the groups, with pairwise analysis revealing a significant increase in mean log C5 (0.4±0.12(SEM); p Conclusion Our results provide the initial evidence of the ability of diphenhydramine to inhibit cough reflex sensitivity in subjects with acute pathological cough.