The contribution of cytochrome P450 2D6 (CYP2D6) to the formation of hydrocodone's active metabolite, hydromorphone, was examined in vitro and in vivo. Human liver microsomes prepared from an individual homozygous for the D6-B mutation of the CYP2D6 gene catalyzed this reaction at a negligible rate. Urinary metabolic ratios of hydrocodone/hydromorphone were highly correlated with O-demethylation ratios for dextromethorphan, an established marker drug of CYP2D6 activity (rs = 0.85; n = 18). The kinetics of hydrocodone after a single oral dose and its partial metabolic clearance to hydromorphone were investigated in five extensive metabolizers of dextromethorphan, six poor metabolizers, and four extensive metabolizers after pretreatment with quinidine, a selective inhibitor of CYP2D6 activity. The mean values for partial metabolic clearance by O-demethylation in the three groups were 28.1 +/- 10.3, 3.4 +/- 2.4, and 5.0 +/- 3.6 ml/hr/kg, respectively. No statistically significant phenotypic differences in physiologic measures were observed. However, over the first hour after dosing, the extensive metabolizers reported more "good opiate effects" and fewer "bad opiate effects" than poor metabolizers and extensive metabolizers in whom CYP2D6 was inhibited by quinidine. These data establish the importance of CYP2D6 in the formation of hydromorphone from hydrocodone and suggest that the activity of this enzyme may limit the abuse liability of hydrocodone.