The most common problem for which a patient seeks medical attention is cough.

Though, there are less scientific data supportive their effectiveness for existing over-the-counter cough remedies. Evidence based observations suggest that the first-generation antihistamine diphenhydramine is categorized as an antitussive by the United States Food and Drug Administration (FDA). In order to substantiate these findings, authors’ of the current study analysed the potential of diphenhydramine in inhibiting cough reflex sensitivity in patients with pathological cough. 

The study enrolled 22 participants with acute viral upper respiratory tract infection (common cold). These patients were randomly assigned into two groups; one with a multicomponent syrup containing diphenhydramine, phenylephrine, in a natural cocoa formulation; dextromethorphan syrup, and other was placebo syrup. They were then undergone determination of cough reflex sensitivity by employing capsaicin challenge on 3 separate days, 2 h after ingesting single doses of study drug (to coincide with peak blood concentrations).  The standard endpoint of cough challenge was concentration of capsaicin inducing ≥5 coughs (C5). 

The results divulged a remarkable difference among both groups, with pairwise analysis demonstrating a significant rise in mean log C5 [0.4 ± 0.55 (SD)] for the diphenhydramine-containing syrup in comparison to placebo.

Thus, based on these findings, it can be concluded that diphenhydramine with its antitussive properties or its potential of inhibiting cough reflex sensitivity can be used successfully in patients with acute pathological cough.

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;37(3):471-474.