SSRI competition with tamoxifen and N-desmethyltamoxifen for CYP2D6, or direct inhibition of CYP2D6 by SSRI, could reduce the production of the tamoxifen metabolites with high receptor-binding affinity, and thereby reduce tamoxifen’s prevention of breast cancer recurrence. The gene discussed is CYP2D6; the disease is breast carcinoma.