Dextromethorphan is a cough suppressant. It affects the signals in the brain that trigger cough reflex. Guaifenesin is an expectorant. It helps loosen congestion in your chest and throat, making it easier to cough out through your mouth. The combination of dextromethorphan and guaifenesin is used to treat cough and chest congestion caused by the common cold, infections, or allergies. Dextromethorphan will not treat a cough that is caused by smoking, asthma, or emphysema.