Robitussin DM's two active ingredients are dextromethorphan, which is a cough suppressant, and guaifenesin, which is an expectorant. Some of Robitussin DM's inactive ingredients are menthol, high fructose corn syrup, purified water, propylene glycol and anhydrous citric acid. To decrease coughing, dextromethorphan interacts directly with the region of the brain that causes coughing. Guaifenesin helps to remove mucus and phlegm from the lungs by loosening and thinning them, explains Drugs.com.
Individuals should treat only periodic coughs with Robitussin DM, and not chronic coughing that occurs as a result of smoking, emphysema or asthma. If a cough does not disappear within seven days of taking Robitussin DM, or occurs alongside a fever, rash or headache, consult a doctor. Additionally, parents should not give Robitussin DM to children under age 4, as it can be harmful. Pregnant or nursing mothers should consult a doctor before using Robitussin DM because it may be passed to the baby and may be harmful, warns Drugs.com.
Dextromethorphan and guaifenesin are available in a variety of different forms, including liquids and capsules, and under a variety of different brand names. Other drugs that contain these active ingredients are DayQuil Mucous Control DM, Delsym Cough and Chest Congestion DM, Broncotron, Phlemex and Mucinex DM, lists Drugs.com.