| Dextropropoxyphene |
Dextropropoxyphene is a weak opioid structurally related to methadone. It is available in many oral preparations, often in conjunction with paracetamol or aspirin. Its combination with paracetamol has been shown to be no more effective than the use of paracetamol alone. Peak systemic concentrations are achieved two hours after oral use, and the elimination half-life is usually about 15 hours, but may be markedly prolonged in the elderly. For the reasons outlined below, use of dextropropoxyphene should be avoided.
Prolonged use of dextropropoxyphene is discouraged, particularly in patients with renal impairment, for the following reasons:
- The extensive first-pass metabolism that occurs initially decreases with chronic use. The resultant increased bioavailability, together with a prolonged elimination half-life of both the drug and its main active metabolite, makes accumulation likely.
- Fixed-dose combinations with paracetamol may be dosed at a frequency that makes dextropropoxyphene accumulation more likely.
- The major metabolite, nordextropropoxyphene, is cardiotoxic.
- Dextropropoxyphene can induce psychological and physical drug dependence and tolerance.
- Chronic use of doses exceeding 800 mg per day has resulted in psychosis and convulsions.
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