• Peter V Dicpinigaitis, Sean Dhar, Amber Johnson, Yvonne Gayle, John Brew, and Wilson Caparros-Wanderley.
• Department of Medicine, Albert Einstein College of Medicine and Montefiore Medical Center, 1825 Eastchester Road, Bronx, NY, 10461, USA, pdicpin@gmail.com.
• Int J Clin Pharm. 2015 Jun 1; 37 (3): 471-4.

BackgroundCurrently 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 United States Food and Drug Administration, to the authors' knowledge it has never been shown to inhibit cough reflex sensitivity in subjects with pathological cough.ObjectiveTo evaluate the effect of diphenhydramine on cough reflex sensitivity.SettingMontefiore Medical Center, an academic medical center in New York City.MethodsTwenty two subjects with acute viral upper respiratory tract infection (common cold) underwent cough reflex sensitivity measurement employing capsaicin challenge on 3 separate days, 2 h after ingesting single doses of study drug (to coincide with peak blood concentrations), administered in randomized, double-blind manner: a multicomponent syrup containing diphenhydramine (25 mg), phenylephrine (10 mg), in a natural cocoa formulation; dextromethorphan (30 mg) syrup; and, placebo syrup. The standard endpoint of cough challenge was used: concentration of capsaicin inducing ≥5 coughs (C5).Main Outcome MeasureEffect on cough reflex sensitivity (C5).ResultsA significant difference (p = 0.0024) was established among groups, with pairwise analysis revealing a significant increase in mean log C5 (0.4 ± 0.55 (SD); p < 0.01) for the diphenhydramine-containing medication versus placebo, but not for dextromethorphan versus placebo.ConclusionsOur results provide the initial evidence of the ability of diphenhydramine to inhibit cough reflex sensitivity in subjects with acute pathological cough. Timing of cough reflex sensitivity measurement may not have allowed demonstration of maximal antitussive effect of dextromethorphan.

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