Abstract: Background: Breast cancer is the most common cancer in women and the second leading cause of mortality among women worldwide. Although there are plenty of treatment options, Tamoxifen remains a cornerstone for treating hormone receptor-positive breast cancer. However, Tamoxifen is still a prodrug that requires enzyme CYP2D6 to be converted to Endoxifen, the most active metabolite. In contrast, Fluoxetine is one of the CYP2D6 inhibitors used for treating anxiety or depression in breast cancer patients.