What is metformin and why is it first-line for type 2 diabetes?

Metformin is an oral antidiabetic drug in the biguanide class that lowers blood glucose by decreasing hepatic glucose production, improving insulin sensitivity in peripheral tissues, and reducing intestinal glucose absorption. It is first-line for type 2 diabetes because it effectively lowers hemoglobin A1C by 1 to 1.5 percent, does not cause weight gain and may promote modest weight loss, does not cause hypoglycemia when used alone, has cardiovascular benefits, and is inexpensive.