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Objective Opioids are recommended by the World Health Organization for moderate to severe cancer pain. Oxycodone is one of the most commonly used opioids and is metabolized in the liver by CYP3A4 and CYP2D6 enzymes. The aim of this cross-sectional study was to assess the relationship between oxycodone pharmacokinetics, pharmacodynamics and the CYP2D6 genotypes “poor metaboliser” (PM), “extensive metaboliser” (EM) and “ultra-rapid metaboliser” (URM) in a cohort of patients with cancer pain. Methods The patients were genotyped for the most common CYP2D6 variants and serum concentrations of oxycodone and metabolites were determined. Pain was assessed using the Brief Pain Inventory (BPI). The EORTC QLQ-C30 was used to assess the symptoms of tiredness and nausea. Cognitive function was assessed by the Mini Mental State (MMS) examination. Associations were examined by analyses of variance (ANOVA) and covariance (ANCOVA), or ordinal logistic regressions with and without covariates. Results The sample consisted of 27 PM, 413 EM (including heterozygotes) and 10 URM. PM had lower oxymorphone and noroxymorphone serum concentrations and oxymorphone to oxycodone ratios than EM and URM. No differences between PM, EM and URM in pain intensity, nausea, tiredness or cognitive function was found. Conclusion CYP2D6 genotypes caused expected differences in pharmacokinetics, but they had no pharmacodynamic consequence. CYP2D6 genotypes did not influence pain control, the adverse symptoms nausea and sedation or the risk for cognitive failure in this study of patients treated with oxycodone for cancer pain.