Snapshot A 44-year-old woman presents status-post lumpectomy and irradiation for a 2.5-cm breast mass. The breast mass is estrogen and progesterone receptor positive. Axillary nodes are negative for cancer. Tamoxifen is added to the patient's treatment regimen. Tamoxifen Mechanism of action estrogen antagonist in breast estrogen agonist in endometrium and bone Clinical use estrogen and progesterone receptor positive breast cancer first-line treatment in premenopausal women to prevent recurrence breast cancer prevention in women high risk for breast cancer Toxicity ↑ risk of of venous thromboembolism ↑ risk of endometrial cancer secondary to agonist activity induces menopause hot flashes Raloxifene Mechanism of acton estrogen antagonist in breast and endometrium agonist in bone Clinical use osteoporosis in menopausal women breast cancer prevention in women high risk for breast cancer Toxicity ↑ risk of of venous thromboembolism induces menopause hot flashes