High interindividual variability in response to tamoxifen treatment of breast-cancer patients calls for a personalized approach to tailor tamoxifen treatment.

Various cytochrome P450 (CYP) enzymes have been proposed, and investigated, to affect the pharmacokinetics and pharmacodynamics of tamoxifen, since tamoxifen is bioactivated to more active metabolites (e.g. endoxifen) by these enzymes.

CYP2D6 genotype showed a clear gene-exposure effect, but can only partially explain interindividual variability. An exposure-response effect remains controversial.

Tailored tamoxifen treatment may not be fully realized through the pharmacogenetics of metabolizing enzymes alone.