The current study assessed 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 reclaimed articles and included studies/clinical trials. Among the total reviewed studies, 11articles 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: Efficacy of diphenhydramine against cough in humans: a review. Pharm World Sci. 2007 Dec;29(6):577-83.