Dextromethorphan is commonly used as a cough suppressant in over-the-counter medications. The efficacy of dextromethorphan in fluctuating cough reflex sensitivity is shown in healthy participants. A significant reduction in induced cough was observed with dextromethorphan when compared with placebo. Both placebo and active treatment improved the subjective data to a comparable degree. 

The objective of this study was to measure the antitussive effects of dextromethorphan when compared with placebo using an established model of smoker’s cough.

A randomized, double-blind placebo-controlled study was conducted in a crossover comparison of 22 mg 0.8 ml (-1) dextromethorphan that was delivered pre-gastrically with corresponding placebo. Objective and subjective measurements of cough were documented. Subjective measures encompassed a day-to-day diary record of cough symptoms and the Leicester questionnaire related to quality of life. Cough frequency was documented by using a manual cough counter. The objective of the dose-response cough challenge related to the measure of cough reflex sensitivity was the citric acid.

The results of the study showed that the dextromethorphan was significantly associated with an upsurge in the concentration of citric acid causing an average of two coughs/inhalation (C2) as compared with placebo, 1-hour post dose by 0.49 mM when compared with placebo 0.24 mM and at 2-hour post dose by 0.57 mM when compared with placebo 0.34 mM. A highly significant improvement in the subjective data was observed when compared with baseline values. But there was no significant variance between placebo and active treatment. There was no association seen between cough sensitivity to citric acid and documented cough counts or symptoms.

The study concluded that the objective measure of cough sensitivity reveals that the dextromethorphan effectively reduces the cough reflex sensitivity.

Ref: Ramsay J, Wright C, Thompson R, et al. Assessment of antitussive efficacy of dextromethorphan in smoking related cough: objective vs. subjective measures. Br J Clin Pharmacol. 2008 May;65(5):737-41.