Tamoxifen 20 mg 50 Tabs is a medication that is used to treat hormonally-responsive breast cancer. It works by blocking the action of the hormone estrogen in the body. It is used in pre- and post-menopausal women, and is available in both pill and liquid form. Common side effects include hot flashes, joint pain, nausea, vomiting, and mood changes. Long-term side effects can include an increased risk of stroke, endometrial cancer, and ovarian cancer.