A Phase III Randomized Trial of Metformin vs Placebo in Early Stage Breast Cancer

This is a multicenter study. Patients are stratified according to hormone-receptor status (estrogen receptor- and/or progesterone receptor- positive vs both receptors negative), body mass index (≤ 30 vs \> 30 kg/m²), HER2 status (positive vs negative), and prior chemotherapy (any vs none). Patients are randomized to 1 of 2 treatment arms. * Arm I: Patients receive oral metformin hydrochloride twice daily (once daily in weeks 1-4). Treatment continues for up to 5 years in the absence of disease progression or unacceptable toxicity. * Arm II: Patients receive oral placebo twice daily (once daily in weeks 1-4). Treatment continues for up to 5 years in the absence of disease progression or unacceptable toxicity. Blood and tumor samples are collected periodically for correlative studies. Patients may complete quality-of-life, physical activity, and diet questionnaires at baseline and at 6, 12, 24, 36, 48, and 60 months. (Sub-set of patients). After completion of study treatment, patients are followed annually.