The cytochrome P450 2D6 (CYP2D6) is a liver enzyme involved in the metabolism of more than 25% of the most frequently used drugs in the clinical field. Variants of the highly polymorphic CYP2D6 gene are associated with altered enzymatic function, ranging from a complete lack of activity to an ultrarapid metabolization of drugs.

This altered CYP2D6 activity causes an inter- individual variability in drug-response. Patients with a defective or over-active CYP2D6 enzyme are either at risk to develop severe adverse events or do not reach the therapeutic window for effective treatment with a specific drug. Hence, pharmacogenetic guidelines recommend a CYP2D6-related dose-adjustment for numerous drugs.