A peripheral mechanism of antitussive activity may be postulated by invoking the presumed effect of guaifenesin on airway mucus. The “hydration hypothesis” proposes that guaifenesin, by increasing the effective hydration of the respiratory tract, maintains the sol layer needed for ciliary clearance and reduces the viscosity of respiratory mucus, thereby further facilitating its removal by natural clearance processes. Perhaps the findings of the present study may be explained by increased mucus volume, or otherwise altered airway mucus, serving as a more effective barrier shielding cough