Dextromethorphan is an antitussive drug that is found in many over-the-counter cold and cough preparations, usually in the form of dextromethorphan hydrobromide. Dextromethorphan is a salt of the methyl ether dextrorotatory isomer of levorphanol, a narcotic analgesic. Dextromethorphan occurs as white crystals, is sparingly soluble in water, and freely soluble in alcohol. The drug is dextrorotatory in water (at 20 degrees Celsius, Sodium D-line) with a specific rotation of +27.6 degrees. Following oral administration, dextromethorphan is rapidly absorbed from the gastrointestinal tract, where it enters the bloodstream and crosses the blood-brain barrier. Dextromethorphan shows high affinity binding to several regions of the brain, including the medullary cough center. The first-pass through the hepatic portal vein results in some of the drug being metabolized into an active metabolite of dextromethorphan, dextrorphan, the 3-hydroxy derivative of dextromethorphan. The therapeutic activity of dextromethorphan is believed to be caused by both the drug and this metabolite. Dextromethorphan is predominantly metabolized by the liver, by various hepatic enzymes. Through various pathways, the drug undergoes (O-demethylation (which produces dextrorphan), N-demethylation, and partial conjugation with glucuronic acid and sulfate ions. The inactive metabolite (+)-3-hydroxy-N-methylmorphinan is formed as a product of DXM metabolism by these pathways. One well known metabolic catalyst involved is a specific cytochrome P450 enzyme known as 2D6, or CYP2D6. A significant portion of the population has a functional deficiency in this enzyme (and are known as poor CYP2D6 metabolizers). As CYP2D6 is the primary metabolic pathway in the inactivation of dextromethorphan, the duration of action and effects of dextromethorphan are significantly increased in such poor metabolizers. Deaths and hospitalizations have been reported in recreational use by poor CYP2D6 metabolizers. -- Wikipedia. This compound is an NMDA receptor antagonist (receptors, N-methyl-D-aspartate) and acts as a non-competitive channel blocker. It is also used to study the involvement of glutamate receptors in neurotoxicity. [PubChem]