Metformin is a well-established ingredient of diabetes management, both as a monotherapy in early stages of type 2 diabetes and as adjunct therapy to virtually every other antihyperglycemic medicine available today. Despite low potency and a long list of contraindications, metformin has remained successful and even expanded experimental use due to its well-established effects on glucose metabolism and, as more recently demonstrated, its benefits on other cardiovascular risk factors. Unlike insulin and secretagogues, metformin does not increase body weight and when used as monotherapy does not likely cause hypoglycemia. The most common adverse effects associated with metformin are mild, transient gastrointestinal symptoms, which are usually self-limiting. These side effects can be minimized by initiating metformin therapy at a low dose and gradually titrating upward, and by taking metformin with meals. Lactic acidosis caused by metformin is rare, and the risk of this complication may be diminished by the observance of prescribing precautions and contraindications that avoid accumulation of metformin or lactate in the body. The many clinical benefits and the lack of safety risks when used with other antihyperglycemic agents have made metformin a preferred combination drug with other oral agents.