Tamoxifen is used in the treatment of women with endocrine responsive breast cancer. It is a prodrug, which is metabolized by liver cytochrome P450 system (CYP) into metabolites with a potent anti-estrogenic activity; CYP2D6 is one of the most important enzymes involved in this metabolic pathway. Low or absent activity of CYP2D6, which can be inherited and/or can occur due to the use of CYP2D6 inhibitors, may impair outcome in women with tamoxifentreated breast cancer. Antidepressants, which can also inhibit CYP2D6, are among the most commonly prescribed drugs in women with breast cancer. Breast cancer patients receiving tamoxifen should not be co-prescribed antidepressants or any other drugs, which are potent inhibitors of CYP2D6.