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Purpose: Codeine is an analgesic drug acting on μ-opioid receptors predominantly via its metabolite morphine formed almost exclusively by CYP2D6. Genetic polymorphisms in CYP2D6 are associated with diminished pain relief and/or severe opioid side effects. In Chinese individuals, CYP2D6*10 is the most common allele with reduced enzyme activity. In this study, we investigated the effect of this allele on the pharmacokinetics of codeine and its metabolites. Method: A blood sample was collected from healthy Mongolian volunteers for CYP2D6 genotyping using a PCR-RFLP assay. A pharmacokinetic study was then carried out in three groups with CYP2D6*1/*1 ( n = 10) , CYP2D6*1/*10 ( n = 10) and CYP2D6*10/*10 ( n = 9) genotypes by collecting serial blood samples for determination of plasma levels of codeine and its metabolites, morphine, morphine 3-glucuronide (M3G) and morphine 6-glucuronide (M6G) before and after a single 30-mg oral dose of codeine phosphate. Codeine and its metabolites were measured by LC-MS/MS. Results: No significant differences were observed in the pharmacokinetic parameters of codeine in the three genotype groups. However, the C and AUC of morphine, M3G and M6G were significantly different between the study groups ( P < 0.05). Compared with the *1/*1 group, the AUC for morphine in the *1/*10 and *10/*10 groups decreased by ratios (95 % CI) of 0.93 (0.26-1.59) and 0.494 (0.135-0.853) respectively. Corresponding ratios for M3G were 0.791 (0.294-1.288) and 0.615 (0.412-0.818) and for M6G were 0.643 (0.39-0.957) and 0.423 (0.267-0.579). Conclusion: This study demonstrates that the CYP2D6*10 allele plays an important role in the pharmacokinetics of the O-demethylated metabolites of codeine after oral administration. [ABSTRACT FROM AUTHOR]