Abstract: Recent preclinical data have demonstrated thatantidiabetic drug metformin can act as an anticancer agent for breast cancer. Epidemiologic evidences show that metformin decreases the incidence of breast cancer and cancer-related mortality in diabetic patients. The metformin treatment also increases complete pathological tumor response rates following neoadjuvant chemotherapy for breast cancer patients. Metformin also displays significant growth inhibitory effects in most types of breast  cancer cell lines  and tumor xenografts in mice. Preclinical data also shows metformin combined with chemotherapy drugs or HER2-targeted drugs or new anticancer drugs has   synergic anti-cancer  effect. Reduction of the insulin/insulin-like growth factor signaling pathway in body and activation of LKB1/AMPK and inhibition  of downstream mTOR  pathway in tumor cells have been found to play the most important role in anti-cancer effect of metformin. Currently, a number of clinical trials are underway in breast cancer patients to evaluate  the effective use value of metformin as a cancer therapy.

Key words: Metformin, Breast neoplasms, Molecular biology