Metformin is a safe, well-tolerated, inexpensive treatment that can be given in addition to current standard-of-care therapies for prostate cancer. Its use might mitigate the deleterious side effects of castration and exert an additional anticancer effect. It will be incorporated in the STAMPEDE trial platform in summer 2016. This will test its true utility as a repurposed treatment for men with high-risk locally advanced or metastatic prostate cancer at first presentation.