Breast cancer is a hormone-dependent tumor in which the growth of cancer cells is regulated by a variety of hormones in the body. Among them, estrogen plays a crucial role in the development of most breast cancers. Elevated estrogen levels are one of the important risk factors for the development of breast cancer. In response to this factor, endocrine therapy for breast cancer has been developed against it. Endocrine therapy is to inhibit the growth of tumor cells by lowering the level of estrogen in the body or inhibiting the effect of estrogen. 1.Who are suitable for endocrine therapy? If the estrogen receptor (ER) and progesterone receptor (PR) of breast cancer cells are tested, if both are positive or either are positive, the patient should receive endocrine therapy after surgery, regardless of age and menstrual status. 2.What are the types of endocrine therapy? (1) Estrogen antagonist therapy. Antagonists are substances that bind to receptors but do not induce biologically active changes. Estrogen antagonists have no effect on hormone levels and block the action of estrogen by binding or blocking estrogen receptors (ER) in cancer cells. This therapy is suitable for patients with estrogen receptor (ER)-positive breast cancer whose cancer has been removed. (2) Estrogen level reduction therapy. It is mainly used to reduce estrogen production and thus estrogen levels through surgical or non-surgical methods. This kind of therapy is suitable for high-risk patients or patients with cancer recurrence months or years after surgery. (1) Commonly used surgical methods include: oophorectomy (an important endocrine therapy for advanced or recurrent breast cancer before menopause or about one year after menopause), adrenalectomy (for patients with effective oophorectomy, if bilateral adrenalectomy is performed again, the condition can be remitted again. (resection), pituitary gland resection. (1) Commonly used therapeutic drugs include: Selective estrogen receptor modulators (estrogen competition inhibitors): inhibit the action of normal estrogen in the body. Representative drug – tamoxifen (triamcinolone), suitable for patients with estrogen receptor-positive breast cancer whose cancer has been removed. Aromatase inhibitors: Effectively reduce the level of estrogen in the body, acting as a “key” to reduce its stimulating effect on cancer cells. Representative drugs – letrozole, anastrozole, exemestane, mainly used for postmenopausal patients with estrogen receptor-positive disease. Hormone receptor modulators: Representative drugs – fulvestrant, which block the action of estrogen by reducing the number of estrogen receptors, remain effective in breast cancer patients who have failed tamoxifen treatment. Others: raloxifene (selective estrogen receptor modulator), progestin (for postmenopausal people, relief treatment for recurrent metastatic breast cancer, still effective in patients who failed tamoxifen treatment), gonadotropin-releasing hormone analog (for premenopausal patients with positive estrogen receptors, and its efficacy can be increased by combining with tamoxifen).