Alfentanil is a validated probe for hepatic, first-pass, and intestinal cytochrome P450 (CYP) 3A activity, using plasma clearances, single-point concentrations and noninvasive pupil diameter change (miosis). Assessing intravenous and oral drug disposition typically requires separate dosing. This investigation evaluated concurrent administration of oral deuterated and intravenous unlabeled alfentanil, to assess both intestinal and hepatic CYP3A, and compare sequential and simultaneous dosing. Alfentanil disposition was evaluated after strong hepatic and/or intestinal CYP3A induction and inhibition by rifampin, ketoconazole, and grapefruit juice. Using plasma alfentanil concentrations and area under the curve, clearance, or single-point concentrations, both simultaneous and sequential dosing provided equivalent results and detected hepatic and intestinal CYP3A induction and inhibition. Miosis better detected CYP3A modulation with sequential vs simultaneous dosing. These results show that concurrent oral deuterated and intravenous alfentanil, administered either sequentially or simultaneously, is an efficient and effective approach to assessing hepatic and intestinal CYP3A activity.