The currently available over-the-counter cough medicines have historically been criticized for lack of scientific data that supports their efficacy. The first-generation antihistamine known as diphenhydramine is classified as an antitussive by the United States Food and Drug Administration but has never been shown to prevent the cough reflex sensitivity in individuals with pathological cough.

The main objective of this trial is to evaluate the effect of diphenhydramine on cough reflex sensitivity. Twenty-two participants with acute viral upper respiratory tract infection i.e., common cold undertook the cough reflex sensitivity measurement test by using a capsaicin challenge on three separate days and 2 hours after ingesting single doses of study drug. The study drug was administered in a randomized and double-blind method as a multicomponent syrup consisting of diphenhydramine 25 mg and phenylephrine (10 mg), in a natural cocoa formulation; along with dextromethorphan 30 mg syrup and placebo syrup. Concentration of capsaicin inducing ≥5 coughs (C5) was the standard endpoint of cough challenge.

The results of the trial showed a significant difference (p = 0.0024) among groups, with pairwise examination that revealed a significant increase in mean log C5 for the diphenhydramine-containing medication versus placebo syrup, but no difference for dextromethorphan versus placebo was seen.

The results of the trial provided the primary evidence of the ability of diphenhydramine to prevent cough reflex sensitivity in individuals with acute pathological cough.

Ref: Dicpinigaitis PV, Dhar S, Johnson A, et al. Inhibition of cough reflex sensitivity by diphenhydramine during acute viral respiratory tract infection. Int J Clin Pharm. 2015 Jun;37(3):471-4. 

The currently available over-the-counter cough medicines have historically been criticized for lack of scientific data that supports their efficacy. The first-generation antihistamine known as diphenhydramine is classified as an antitussive by the United States Food and Drug Administration but has never been shown to prevent the cough reflex sensitivity in individuals with pathological cough.

The main objective of this trial is to evaluate the effect of diphenhydramine on cough reflex sensitivity. Twenty-two participants with acute viral upper respiratory tract infection i.e., common cold undertook the cough reflex sensitivity measurement test by using a capsaicin challenge on three separate days and 2 hours after ingesting single doses of study drug. The study drug was administered in a randomized and double-blind method as a multicomponent syrup consisting of diphenhydramine 25 mg and phenylephrine (10 mg), in a natural cocoa formulation; along with dextromethorphan 30 mg syrup and placebo syrup. Concentration of capsaicin inducing ≥5 coughs (C5) was the standard endpoint of cough challenge.

The results of the trial showed a significant difference (p = 0.0024) among groups, with pairwise examination that revealed a significant increase in mean log C5 for the diphenhydramine-containing medication versus placebo syrup, but no difference for dextromethorphan versus placebo was seen.

The results of the trial provided the primary evidence of the ability of diphenhydramine to prevent cough reflex sensitivity in individuals with acute pathological cough.

Ref: Dicpinigaitis PV, Dhar S, Johnson A, et al. Inhibition of cough reflex sensitivity by diphenhydramine during acute viral respiratory tract infection. Int J Clin Pharm. 2015 Jun;37(3):471-4.