CYP2D6 and breast cancer: To overcome an increased risk for breast cancer recurrence in CYP2D6 variant patients two different approaches have been suggested (i) CYP2D6 phenotype-adjusted tamoxifen dosage in CYP2D6 IMs and PMs to achieve similar endoxifen Css compared with EMs and (ii) direct administration of the active metabolite endoxifen which ensures independence from the CYP2D6 polymorphism.