Narcotic (opioid) analgesic agents are extensively metabolized by the
liver,
and several of them (e.g., codeine, hydrocodone, meperidine, methadone,
morphine, propoxyphene) have active metabolites that are further converted to inactive substances. The serum concentrations of these agents and their metabolites may be increased and the half-lives prolonged in patients with impaired hepatic function.