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The formation of morphine from codeine and ethylmorphine is mainly mediated by the polymorphic enzyme CYP2D6. The objective of this study was to investigate whether CYP2D6 poor metabolizers (PM) and CYP2D6 extensive metabolizers (EM) would respond differently during testing for opiate drugs of abuse in urine after intake of these drugs. Five PM and five EM of dextromethorphan were administered single oral doses of codeine (25 mg) and ethylmorphine (25 mg), and the urinary excretion of parent compounds and selected metabolites was observed for 72 hours. Analysis was performed with GC-MS after hydrolysis of the glucuronide conjugates. Selected urine samples were screened for the presence of opiates by the Abbott ADx immunoassay method. The results from one PM and one EM were excluded because of technical analytical problems. EM excreted significantly more morphine than PM after intake of both codeine (6.5% vs. 1.1% of the dose; p < 0.05) and ethylmorphine (11.0% vs. 3.0% of the dose; p < 0.05). Screening results were positive significantly longer for EM than for PM after codeine intake (mean, 33 hours vs. 17 hours; p < 0.05), and the same trend, albeit nonsignificantly, was noted for ethylmorphine (mean, 33 hours vs. 24 hours). Regardless of CYP2D6 phenotype, significantly more morphine was formed after intake of ethylmorphine than after intake of codeine (7.0% vs. 3.8% of the dose; p < 0.05). There were high correlations between dextromethorphan metabolic ratios and the ratios of codeine to morphine, ethylmorphine to morphine, norcodeine to normorphine, and norethylmorphine to normorphine (r = 0.80 to 0.92; p = 0.030 to 0.001). Although this study should be interpreted with caution because of the few subjects included and the single-dose design, it demonstrates that the CYP2D6 phenotype clearly affects the results when testing for opiates in urine after intake of codeine and ethylmorphine.