Tamoxifen (TAM) is a selective estrogen receptor modulator employed in the treatment of breast cancer. It is a prodrug with a complex metabolic pathway involving several phase I and II metabolic enzymes. TAM is metabolized by cytochrome P450 (CYP) enzymes to N-desmethyltamoxifen (NDM), 4-hydroxytamoxifen (4OHT) and endoxifen (END) with 4OHT and END being the active metabolites of TAM. CYP2D6 and CYP3A4/5 comprises the major CYP isoforms mediating the metabolism of TAM although other CYP isoforms also play a role. Polymorphisms present in genes encoding these CYP enzymes may influence the metabolism and pharmacokinetics of TAM and its metabolites.