OBJECTIVE We studied the influence of three factors on drug disposition: genetic polymorphism, impaired renal excretion of drug metabolites, and the possible elimination by hemodialysis (HD), using codeine as a model drug. METHODS Based on the genotyping of three CYP2D6 polymorphisms in 228 HD patients, nine extensive metabolizers (EMs) and two poor metabolizers (PMs) were given a single oral dose of 50 mg codeine phosphate. Plasma concentrations of its metabolites codeine-6-glucuronide (C6G), morphine-3-glucuronide (M3G), and morphine-6-glucuronide (M6G) were determined after 2, 4, 6, 8 and 24 h (beginning of the HD session) and again after 4 h of HD (28 h). Codeine metabolites in plasma were quantitated by liquid chromatography-mass spectrometry (LC-MS). RESULTS The concentrations of C6G in plasma were high and similar in EMs and PMs. Two hours after the codeine intake, the mean concentration of M3G was 210 nM in EMs vs. 3.5 nM in PMs. The M6G metabolite concentrations could be quantitated in EMs but were below the limit of quantification in PMs (<1 nM). All three codeine metabolites/glucuronides remained unchanged or even increased until the start of HD, and thereafter, the concentrations decreased dramatically during the HD procedure. CONCLUSIONS Formation of the codeine metabolites M3G and M6G was dependent on the CYP2D6 genotype, as previously shown in healthy individuals. Elimination of glucuronides in these patients was absent until HD was performed. These factors need to be taken into consideration when drugs metabolized by CYPs are prescribed in HD patients.