This combination medicine is used for the temporary treatment of cough, catarrh, and nasal congestion caused by the common cold, flu, allergies, hay fever, or other respiratory illnesses (e.g., sinusitis, bronchitis). Guaifenesin is an expectorant that helps loosen and dissolve phlegm in the lungs, making it easier to cough up. Dextromethorphan is a cough suppressant that acts on a certain part of the brain (the cough center), reducing the urge to cough. This product also contains a decongestant to help relieve nasal congestion.