 Metformin is a widely used anti-diabetic drug that has recently been re-proposed for use in treating cancer. It has been shown to reduce the risk of cancer and decrease cancer-related mortality in patients with diabetes. The exact mechanisms behind its anti-cancer properties remain unclear, but it appears to work by either directly affecting cancer cells or by changing the bodies over all metabolism. This review provides an update on the current understanding of how metformin works against cancer, discusses the underlying mechanisms, and proposes potential applications for re-proposing metformin for cancer therapy. This article was authored by Yu Hua, Yu Xun, Yiran Yao, and others.