The over-the-counter cough medicines that are currently available are criticized for deficiency of scientific data that supports their efficacy. Even though the first-generation antihistamine i.e., diphenhydramine is classified as an antitussive by the United States Food and Drug Administration, but to the authors information it has never shown to prevent the cough reflex sensitivity in individuals with pathological cough.

The objective of the study is to evaluate the effectiveness of diphenhydramine on cough reflex sensitivity. Twenty-two participants with acute viral upper respiratory tract infection such as common cold undertook cough reflex sensitivity measurement. They took this cough reflex measurement by using a capsaicin challenge on three separate days, 2 hours after taking the single doses of study drug, which was administered in a randomized and double-blind manner. A multicomponent syrup that contains diphenhydramine (25 mg) and phenylephrine (10 mg), in a natural cocoa formulation along dextromethorphan 30 mg syrup and placebo syrup. The average endpoint of cough reflex challenge was concentration of capsaicin that induces ≥5 coughs (C5).

The results of the study showed that a significant difference was observed among groups, with pairwise study that revealed a significant increase in mean log C5 for the diphenhydramine-containing medicine versus placebo. There was no significant difference seen for dextromethorphan versus placebo.

The results of the study also provided the initial evidence of the ability of diphenhydramine to inhibit cough reflex sensitivity in individuals with acute pathological cough. Sensitivity measurement of cough reflex might not have allowed the demonstration of maximal antitussive effect of dextromethorphan.

Source: Dicpinigaitis PV, Dhar S, Johnson A, Gayle Y, Brew J, Caparros-Wanderley W. Inhibition of cough reflex sensitivity by diphenhydramine during acute viral respiratory tract infection. Int J Clin Pharm. 2015 Jun;37(3):471-4.