The most serious side effect of tamoxifen is that it can cause endometrial thickening. Mild thickening of the uterine lining can be observed, and if it is significantly thickened, even causing atypical endometrial hyperplasia and irregular vaginal bleeding, it is recommended that tamoxifen be discontinued, and at this time, it can be changed to subcutaneous injections of endocrine denervation medications such as goserelin, as well as oral aromatase inhibitors, such as letrozole, anastrozole, or exemestane, and to pay attention to sun exposure, and to take calcium by mouth to prevent severe bone loss and to prevent Osteoporosis. Oophorectomy with aromatase inhibitors is also an option if the patient does not plan to have another child.