CYP2D6 and breast cancer: In this context, Van der Merwe et al. [28] recommended CYP2D6 genotyping in breast cancer patients who (1) receive tamoxifen treatment; and (2) are at increased recurrence risk due to a family history of breast cancer/BRCA-mutation positivity; or (3) are required to take potential competing antidepressants that may inhibit CYP2D6 enzyme function.