Metformin, an oral biguanide agent used for the treatment of non-insulin-dependent diabetes mellitus, is now prescribed to more than 120 million people worldwide. Its glucose lowering effects result from both inhibition of liver gluconeogenesis and increased insulin sensitivity in peripheral tissue. Metformin has limited adverse effects with little or no risk of hypoglycemia in healthy, nondiabetic controls. In addition to its anti-diabetic properties, metformin has demonstrated both chemopreventative and therapeutic effects in both prostate and breast cancer. Jiralersprong et al reported that diabetic patients with breast cancer receiving metformin had a 24% complete pathological response rate to neoadjuvant chemotherapy compared to only 8% of those in the non-metformin group. More recently Joshua et al reported reduced tumor Ki-67 rate as well as significant reductions in fasting glucose, insulin growth factor 1 and BMI (body mass index) in prostate cancer patients receiving neoadjuvant metformin prior to radical prostatectomy. Large epidemiological studies consistently have shown substantially lower incidence of cancer occurrence and death in diabetic patients taking metformin compared to those receiving other therapies. Most recently, a retrospective study of patients with ovarian cancer found that 5-year disease-specific survival was significantly better for diabetic patients who took metformin than for those who did not (67% vs 47%; P = .007). Based on these important observational studies, there are currently several, ongoing prospective studies evaluating metformin in nondiabetic patients with both early stage and late stage breast cancer and prostate cancer, respectively.