Epidemiologic evidence suggests that insulin resistance and high insulin levels worsen outcomes for breast cancer patients. Metformin may reduce recurrences of breast cancer through multiple mechanisms. It helps to decrease insulin levels by decreasing insulin resistance and gluconeogenesis in the liver. Metformin also acts intracellularly on cancer cells to limit cell proliferation and increase apoptosis. Various studies have indicated that the incidence of breast cancer may be lower in diabetic patients treated with metformin, and large-scale trials are ongoing for use in the adjuvant setting (Jacobs, 2010).