The present study evaluated the effectiveness of diphenhydramine against cough due to respiratory infection or irritation in patients without comorbidities. The researchers searched Medline (1966-2005), Embase (1980-2005), Cochrane and references from retrieved articles and included studies/clinical trials using keywords such as diphenhydramine (OR dimenhydrinate), antitussive agents, cough. These studies were then blinded to study origin selected studies, inclusion criteria was diphenhydramine monotherapy against placebo, double-blinded, randomized, clinical trial, intention-to-treat, dropout information. The blinded reviewers assessed the selected studies on a quality scale.

Among the total reviewed studies, 11 articles were recognized, 7 were rejected, remaining 4 articles that were included in the analysis and scored 20, 21, 25 and 26 out of a maximum of 32. A total of 162 people were observed, 65 on diphenhydramine, 63 on placebo and 34 in a crossover setting in these selected studies. The results from the crossover studies showed remarkable efficacy; 27-56% reduction in 20 healthy volunteers and 21-26% reduction in 13 patients.

However, the evidences are still limited assessing the efficacy of diphenhydramine in the treatment of cough, diphenhydramine in crossover studies has shown its therapeutic potential in the treatment of cough.

Source: Björnsdóttir I, Einarson TR, Gudmundsson LS, Einarsdóttir RA. Efficacy of diphenhydramine against cough in humans: a review. Pharm World Sci. 2007 Dec;29(6):577-83.