In recent years, the prevalence of breast cancer and type 2 diabetes has been increasing. Many patients have both diabetes and breast cancer, and some patients are even diagnosed with type 2 diabetes while hospitalized for breast cancer. Most patients believe that the treatment of diabetes and breast cancer are not related to each other, and some health care professionals even hold this view. In fact, recent studies have found that metformin, one of the drugs used to treat type 2 diabetes, has some anti-tumor activity. Firstly, epidemiological studies have found that the incidence of malignant tumors is lower in type 2 diabetic patients taking metformin than in those not taking metformin. type 2 diabetes is a high risk factor for the development of many malignant tumors, including breast cancer, and is also a factor in the poor prognosis of malignant tumors. However, patients taking metformin have a 31% lower risk of developing various malignancies. In the case of breast cancer, a Danish study showed that taking metformin reduced the risk of breast cancer by 23% compared to diabetic patients who did not take metformin. A Swiss study showed that long-term metformin use reduced the risk of breast cancer by 56%. A subsequent study in the United States found that patients with type 2 diabetes who took metformin concurrently with neoadjuvant chemotherapy significantly improved the effectiveness of chemotherapy. The study found that after completion of neoadjuvant chemotherapy, tumors disappeared completely in 8% of diabetic patients not taking metformin, compared to 24% of diabetic patients taking metformin. The rate of complete tumor disappearance in patients without combined diabetes was 16%. Therefore, the researchers suggest that metformin should be added to chemotherapy for breast cancer patients. Studies at the West China Hospital in China have shown that metformin during chemotherapy for patients with advanced non-small cell lung cancer can enhance the therapeutic effect. In vitro tests have also shown that metformin has the function of anti-tumor cell growth and invasion, and metformin also has the function of anti-tumor stem cells. It is currently believed that tumor stem cells are cells that cannot be killed by conventional chemotherapeutic drugs and may be the cause of tumor recurrence and metastasis. In addition, in vitro tests have shown that metformin also has a killing effect on triple-negative breast cancer (i.e. estrogen receptor, progesterone receptor and Her-2 are all negative) cells. In view of this, prospective clinical trials of adjuvant treatment of breast cancer with metformin have been conducted overseas. The trial is still in the ongoing phase and is inconclusive. Based on the above reasons, all breast cancer patients with concomitant type 2 diabetes should consult an endocrinologist and take metformin therapy, which is beneficial not only for the treatment of diabetes but also for the treatment of breast cancer.