Which is not a common use for expectorants?

Expectorants are primarily designed to increase the amount of respiratory tract fluid and reduce the viscosity of mucus, making it easier to expel from the lungs. Their common uses include relieving cough associated with bronchitis, thinning mucus to assist with clearer airways, and facilitating mucus clearance by promoting the expulsion of phlegm.

The option identifying preventing cough due to irritants as a non-common use for expectorants is accurate because expectorants do not specifically target the prevention of cough; rather, they help alleviate cough symptoms already present by aiding in mucus clearance. In cases of cough induced by irritants, other classes of medications like antitussives may be more appropriate for suppressing the reflex action of coughing. Thus, the correct answer highlights an understanding of the primary roles of expectorants in pharmacotherapy for respiratory conditions.