After breast cancer surgery, there are still some cancer cells left in the body, leaving the hidden danger of breast cancer recurrence. The purpose of endocrine therapy is to reduce the production of estrogen in our body or inhibit estrogen binding to breast cancer cells, thus inhibiting the growth and proliferation of these cancer cells, preventing metastasis and recurrence of breast cancer after surgery and prolonging survival time. There is a significant recurrence peak 1-3 years after breast cancer surgery, and the most common form of recurrence is distant metastasis, which accounts for 3/4 of all recurrences, while other forms of recurrence include local recurrence and distant metastasis. Prevention of distant metastases is important because the survival rate is low after the occurrence of distant metastases, with a 5-year survival rate of only 22% and a 10-year survival rate of only 5%.