['This study was designed to investigate whether the in vivo metabolism of tramadol was influenced by CYP2D6 polymorphism.', 'The extent of tramadol O- and N-demethylation was calculated by determining the amounts of tramadol and O- and N-desmethyltramadol in 24 h urine after ingestion of a test dose of tramadol. The O- and N-demethylation rates were calculated by dividing the 24-h urinary excretion amount of tramadol by that of O- and N-desmethyltramadol. Volunteers were phenotyped for CYP2D6 polymorphism using sparteine as an in vivo probe.'] Does polymorphic CYP2D6 mediate O-demethylation of the opioid analgesic tramadol?