Metformin, sold under the brand name Glucophage among others, is the first-line medication for the treatment of type 2 diabetes, particularly in people who are overweight. It is also used in the treatment of polycystic ovary syndrome. It is not associated with weight gain and is taken by mouth. It is sometimes used as an off-label augment to attenuate the risk of weight gain in people who take antipsychotics as well as phenelzine.

Metformin is generally well tolerated. Common adverse effects include diarrhea, nausea, and abdominal pain. It has a low risk of causing low blood sugar. High blood lactic acid level is a concern if the medication is used in overly large doses or prescribed in persons with severe kidney problems. It is not recommended in those with significant liver disease. Metformin is a biguanide antihyperglycemic agent. It works by decreasing glucose production by the liver, by increasing the insulin sensitivity of body tissues, and by increasing GDF15 secretion, which reduces appetite and caloric intake.

Metformin was discovered in 1922. French physician Jean Sterne began the study in humans in the 1950s. It was introduced as a medication in France in 1957 and the United States in 1995. It is on the World Health Organization's List of Essential Medicines. Metformin is the most widely used medication for diabetes taken by mouth. It is available as a generic medication. In 2019, it was the fourth-most commonly prescribed medication in the United States, with more than 85 million prescriptions.