A detailed discussion of the absorption, distribution, and elimination of opioids is beyond the scope of this chapter. However, when clinically relevant, details of absorption and elimination of specific opioids are given. Knowledge of duration of action and elimination half-life is useful and is provided in Table.161-1.
Most opioids are more effective when given parenterally than orally, owing to variable but significant first-pass effect. Opioids with good oral potency are codeine, oxycodone, levorphanol, and methadone.
The metabolism of codeine, morphine, propoxyphene, oxycodone, meperidine, and methadone is mostly hepatic. The hepatic metabolites may be pharmacologically active. Concurrent use of benzodiazepines, barbiturates, and alcohol is common in the opioid abuser either because of their additive effect or their capacity to inhibit hepatic metabolism, especially the metabolism of methadone. Cyclic antidepressants and propoxyphene also decrease methadone metabolism.
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