Tamoxifen, widely used to treat breast cancer,  is a prodrug that is extensively metabolised via Cytochrome P450 (CYP2D6) isoenzymes in the liver to active metabolites. Concomitant use of drugs that are potent or moderately potent inhibitors of the CYP2D6 enzyme should be avoided whenever possible. 

 IMSN Safety Alert on Tamoxifen & SSRI/SNRI Antidepressants: Potential for Significant Drug Interaction