In the present study, we investigated the pharmacokinetics profile and the influence of CYP2D6, CYP2C9, CYP2C19, CYP3A5, POR, ABCB1 and UGT2B15 polymorphisms on the plasma level of tamoxifen and its active metabolites in Ethiopian breast cancer patients receiving adjuvant tamoxifen. This evidence concerns the gene CYP3A5 and breast cancer.