Dextromethorphan hydrobromide (DXM for short) is an antitussive drug that is found in many over-the-counter cold remedies and cough syrups. In considerably higher doses, DXM is a powerful psychedelic drug. (Specifically, it is a dissociative drug. The dissociatives are a major subclass of the psychedelic drugs.) It is not physically addictive, but may be psychologically addictive (see below).

GoldBamboo

... DEXTROMETHORPHAN (Benylin®, Delsym®, Pertussin®, Robitussin®) helps to relieve a persistent cough caused by colds or the flu. Do not use dextromethorphan for treatment of a chronic cough that is due to smoking, asthma, emphysema, or problems that cause a large amount of phlegm. Generic dextromethorphan is widely available in a variety of non-prescription medicines, in capsules, lozenges, syrups, extended-release oral suspensions, and chewable tablets. ...
Drug Digest - more...

Dextromethorphan Systematic (IUPAC) name D-(+)-3-methoxy-17-methy ...
Wikipedia - [full article]

Resources