Not Medical Advice: Dextromethorphan, the "DM" in Mucinex DM, is a cough suppressant, but it is also used as a recreational drug, more commonly with teens as it is easily obtained. In a 2011 survey, approximately 5 percent of teens had reported abusing the drug in the past year.
When it is taken at doses higher than recommended, dextromethorphan is classified as a hallucinogen and acts similar to phencyclidine (PCP). Side effects include disassociation, nausea, vomiting, blurred vision, sweating, fever, and shallow breathing. Recreational use of any medication is extremely dangerous.