According to the theory of “window of action” of estrogen, the treatment of endometriosis can be achieved by maintaining the blood estradiol level below 50 pg/ml, and further reduction of estradiol level does not increase the therapeutic effect, but rather aggravates the side effects. After treatment with GnRH-a alone, blood estradiol levels often fall below 20 pg/ml. Therefore, exogenous addition of small doses of estrogen to raise the body’s estrogen and maintain it at the “window” level can reduce the side effects of the drug without affecting its efficacy. This method of adding small doses of estrogen is called “add-back” therapy. In order to counteract the endometrial irritation caused by the continuous application of estrogen, patients with a uterus can supplement estrogen with a moderate amount of progestin. Commonly used reverse supplementation regimens are: Bemelax 0.3mg to 0.625mg once daily + Amgen Progesterone 2mg to 4mg, or Levitra 1.25mg to 2.5mg once daily. Estrogen is essential for maintaining the functions of many tissues and organs in the body, and the sensitivity of different tissues and organs to estrogen varies, which means that the minimum estrogen concentration for maintaining the functions of different tissues and organs is different. For example, menopausal symptoms such as hot flashes and sweating can occur when blood estradiol levels fall below 30 pg/ml, while bone loss can only occur when blood estradiol levels fall below 20 pg/ml. Among estrogen-related diseases, the sensitivity to estrogen varies from disease to disease. For example, breast cancer is most sensitive to estrogen, and the growth of breast cancer cells can be stimulated when the level of estradiol in blood reaches 20 pg/ml; uterine fibroids are also sensitive to estrogen, and the growth of fibroids can be stimulated when the level of estradiol in blood reaches about 30 pg/ml. However, it takes a blood estradiol level of 40 to 50 pg/ml to cause the growth of endometriosis. Therefore, maintaining estradiol levels between 30 and 50 pg/ml during endometriosis medication can achieve the goal of not stimulating the growth of ectopic foci while avoiding menopausal symptoms and bone loss, which is known as the “window of estrogen action theory”.