Dextromethorphan is a cough suppressant that is available in over-the-counter cough medications. Dextromethorphan (DXM) is the active ingredient in most OTC cough medicines. Approved by the U.S. Food and Drug Administration in the 1950s, DXM is the most widely used cough suppressant ingredient in the United States. When taken according to labeling instructions, medicines that contain DXM are safe and effective. However, when taken in excessive amounts higher than recommended doses, DXM can produce dangerous side effects.Because it is a legal drug, it can be purchased by anyone, even a preteen, except in California. There, a minor cannot purchase a cough medication containing dextromethorphan (DXM).