Bleeding from endocrine therapy for breast cancer, usually vaginal bleeding, is caused by drug-conducted inhibition of estrogen. Endocrine therapy for breast cancer is mainly divided into premenopausal and postmenopausal, the representative drug of premenopausal is tamoxifen, and the endocrine drug of postmenopausal is aromatase inhibitor, which blocks the binding of estrogen to the receptor or inhibits the production of estrogen, so as to achieve the effect of inhibiting the growth of tumor cells. It is because of its estrogen-inhibiting effect, which in turn tends to lead to menstrual disorders, manifested as vaginal bleeding. There are many factors that contribute to bleeding in breast endocrine therapy, and you should seek medical attention in the hospital to identify the cause and treat the symptoms.