Cough results from a complex interaction between receptors in the airway and the central nervous system; this interaction is poorly understood. Glyceryl ether of guaiacol, commonly known as guaifenesin, is present in multiple cough and cold preparations. This compound has been labeled an expectorant because it is thought to relieve cough by thinning sputum and increasing its production. Guaifenesin is the only expectorant that has been considered effective by the U.S. Food and Drug Administration. Despite its broad use in the treatment of cough, multiple studies have found inconsistent results supporting the effectiveness of guaifenesin as an expectorant or cough suppressant. Little information is available concerning the impact of guaifenesin on the sensitivity of the cough reflex. Dicpinigaitis and Gayle conducted a randomized, placebo-controlled, double-blind study to assess the efficacy of a single dose of guaifenesin on cough reflex sensitivity.
The trial involved a group of persons with acute viral upper respiratory tract infection (URI) and a group of healthy persons. The participants were nonsmokers, did not have a history of pulmonary disorders, and had no symptoms suggestive of gastroesophageal reflux disease. Within the previous four weeks, the healthy participants had no URI symptoms, seasonal allergies, or postnasal drip. Participants with URI took no medications to control symptoms within at least 24 hours of the study.
Each participant received a single 400-mg dose of guaifenesin or placebo one to two hours before undergoing a capsaicin challenge to induce cough. The capsaicin challenge was performed with incremental increases in concentration until it induced five or more coughs within 15 seconds of inhalation. Spirometry data were collected before the challenge. The main outcome was the concentration of capsaicin required to induce five or more coughs. The results included comparing the effects of guaifenesin and placebo among the patients with URI and the healthy participants.
The URI and healthy groups each contained 14 persons. All participants were induced to cough during the capsaicin challenge test. The spirometry results did not differ between persons who received guaifenesin and persons who received placebo. Among patients with URI, those who received guaifenesin required significantly higher concentrations of capsaicin to induce cough compared with the placebo group. In seven of the 14 participants who received guaifenesin, the concentration needed to induce cough was double that of the placebo group. Among the healthy persons, no differences occurred between the placebo and guaifenesin groups with regard to the concentration of capsaicin that induced five or more coughs.
The authors conclude that guaifenesin inhibits the cough reflex in patients with URI. This action occurs because the cough receptors are transiently hypersensitive secondary to the URI. They add that this effect was not present in healthy persons.