Metformin is a first-line oral drug for treating type 2 diabetes that works by decreasing glucose production in the liver and intestinal absorption of glucose. It has been widely used for over 60 years and is prescribed to over 120 million people worldwide. Metformin is well-tolerated and has few side effects besides mild gastrointestinal issues, though it is contraindicated in patients with kidney or liver disease. Its mechanisms of action include increasing insulin sensitivity, decreasing appetite, and improving lipid profiles.