Which drug is classified as an expectorant?

Guaifenesin is classified as an expectorant because its primary function is to aid in the clearance of mucus from the airways, making it easier to cough up and expel. It works by increasing the volume and hydration of respiratory tract secretions, thereby thinning the mucus in the airways. This action enhances the ability to clear secretions from the lungs and contributes to alleviating symptoms associated with cough and congestion, which is why it's commonly found in over-the-counter cough and cold medications.

Cromolyn is not an expectorant; rather, it is a mast cell stabilizer primarily used for preventing asthma attacks by inhibiting the release of inflammatory mediators. Theophylline is a bronchodilator used primarily for asthma and COPD that aids in airway relaxation but does not specifically function as an expectorant. Montelukast is a leukotriene receptor antagonist, also used primarily in asthma management and allergy relief but does not have the properties of an expectorant. Thus, guaifenesin stands out as the appropriate choice for its role in mucus clearance and expectoration.