Dextromethorphan is the dextrorotatory form of levorphanol with a methoxy group on the 3-position (Fig. 24.7). It is available in more than 140 over-the-counter (OTC) cough and cold formulations.69 Evidence-based reviews have been unable to conclude that it is more effective than placebo in reducing cough.61,70 Like (+) and (—) levorphanol, (+) dextromethor-phan is a potent NMDA antagonist and, in higher than recommended doses, has the potential for causing dissociative anesthetic effects similar to ketamine or phencyclidine (PCP). The OTC status and availability of pure dextromethorphan powder online has contributed greatly to its abuse in recent years. DAWN reports that in 2004, there were approximately 12,500 emergency room visits involving dextromethorphan with 44% of those involving abuse of the drug.71 The 2006 National Survey on Drug Abuse report shows that nearly 1 million persons aged 12 to 25 years (1.7%) misused OTC cough and cold medications in the past year.69
Dextromethorphan's ability to antagonize the NMDA receptor has led to its use to treat phantom pain, diabetic neuropathy, and postoperative acute pain.72,73
Structural simplification of the morphine ring system further, by removing the C ring of the morphinan structure, yields the benzomorphans also referred to as the benzazocines.
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