Propoxyphene (dextropropoxyphene; Darvon) is struc-turally related to methadone but is much less potent as an analgesic. Compared with codeine, propoxyphene is approximately half as potent and is indicated for the treatment of mild pain. It is not antipyretic or antiin-flammatory like aspirin and is less useful than aspirin in most cases of mild pain. Toxicity from propoxyphene, especially in combination with other sedatives, such as alcohol, has led to a decrease in its use. Death following ingestion of alcohol in combination with propoxyphene can occur rapidly (within 20 minutes to 1 hour). The drug is not indicated for those with histories of suicide or depressive illnesses.
Like meperidine, propoxyphene has an active metabolite, norpropoxyphene, that is not analgesic but has excitatory and local anesthetic effects on the heart similar to those of quinidine. Use of the drug during pregnancy is not safe. Teratogenic effects have been ob-served in newborns, as have withdrawal signs at birth. As with morphine, propoxyphene requires adequate he-patic and renal clearance to prevent toxicity and drug accumulation. It is thus contraindicated in the elderly patient and those with renal or liver disease.
Propoxyphene interacts with several drugs. The use of sedatives in combination with propoxyphene can be fatal. In addition, the metabolism of the drug is in-creased in smokers due to induction of liver enzymes. Thus, smokers may require a higher dose of the drug for pain relief. Propoxyphene enhances the effects of both warfarin and carbamazepine and may increase the tox-icity associated with both drugs, such as bleeding and se-dation, respectively.
The abuse liability of propoxyphene is low because of the extreme irritation it causes at the site of injection. Oral use is the preferred route of administration for this reason.
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