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Recent studies showed that metformin, the most commonly used first-line oral drug for the treatment of type 2 diabetes (T2DM), could prevent and treat various cancers. And many preclinical studies showed that metformin can inhibit the growth of cancer cells in vitro and in vivo. Moreover, multiple evidence suggested that metformin inhibited cancer invasion and metastasis, which could improve the prognosis of cancer patients administrated with metformin. In this summery, we aimed to discuss the possible relationship between breast cancer and metformin use in T2DM from the risk, incidence, response to therapy, prognosis, resistance, and post-therapeutic relapse in cancers. Although some of the controversal studies described, there is no doubt that metformin use in T2DM is benifical for breast cancer.
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