Metformin is used to treat type 2 diabetes and insulin resistance in polycystic ovary syndrome. It was FDA-approved for weight loss almost thirty years ago. Weight loss is a known side-effect. How metformin causes weight loss is not fully understood, but it may involve changes in the appetite centers in the brain, a decrease in food intake, decreased glucose production in the liver, or improved insulin sensitivity. Weight loss is likely due to decreased caloric intake rather than increased energy expenditure. Weight loss in metformin studies varies from 6 pounds to 13 pounds (+/- 15 pounds).

Because the weight loss associated with metformin is modest and inconsistent, the FDA has not approved it as a weight-loss agent, but it continues to be used off-label. The 2015 Endocrine Society Practice Guideline on Pharmacology for Obesity does not recommend the use of metformin as monotherapy for obese patients without metabolic complications such as diabetes. The 2016 AACE/ACE guidelines on obesity management recommend the use of metformin in patients with obesity and evidence of prediabetes or insulin intolerance that does not respond to lifestyle medications or other anti-obesity medications. 

Metformin is also being investigated as an anti-aging medication.