Metformin, an inexpensive oral agent commonly used to treat type 2 diabetes, has been garnering increasing attention as a potential anti-cancer agent. Preclinical, epidemiologic, and clinical evidences suggest that metformin may reduce overall cancer risk and mortality, with specific effects in breast cancer. The extensive clinical experience with metformin, coupled with its known (and modest) toxicity, has allowed the traditional process of drug evaluation to be shortened. We review the rationale for a modified approach to evaluation and outline the key steps that will optimize development of this agent in breast cancer, including discussion of a Phase III adjuvant trial (NCIC MA.32) that has recently been initiated.