Metformin is the only medication in the category called biguanides approved by the U.S. Food and Drug Administration (FDA). It's been available in the United States since the mid-1990s, when it received FDA approval. Initially available under the brand name Glucophage because it was under patent, metformin is now widely available as a generic. Its main action is to decrease the overproduction of glucose by the liver. This action helps lower blood glucose levels particularly after fasting but also after meals. Metformin also increases the uptake of glucose by your muscles. All in all, metformin decreases insulin resistance and improves insulin sensitivity, thereby helping the insulin your body still makes work more effectively.
Since its approval, metformin has become the preferred first-approach blood glucose-lowering medication to treat type 2 diabetes, replacing sulfonylureas, such as glipizide and glyburide. Today, both the American Diabetes Association and the American Association of Clinical Endocrinologists recommend that people with type 2 diabetes start on metformin when they are diagnosed because of its action to decrease insulin resistance.
There are two other side benefits of metformin over the sulfonylurea category of medications: It does not seem to cause weight gain (in fact, you may even lose a few pounds), and it does not cause low blood sugar when used without other blood glucose-lowering medications that can.
Metformin is approved for use with a number of the other blood glucose-lowering medications, such as insulin, TZDs (Actos, Avandia), sulfonylureas, DPP-4 inhibitors, and GLP analogs (Byetta, Victoza). Several combination pills with metformin and another blood glucose-lowering oral medication are also available.