Metformin is primarily used for type 2 diabetes but is increasingly being used in polycystic ovary syndrome
The UK Prospective Diabetes Study, a large clinical trial performed in the 1980-90s, provided evidence that metformin reduced the rate of adverse cardiovascular outcomes in overweight patients with type 2 diabetes relative to other antihyperglycemic agents. However, accumulated evidence from other and more recent trials has reduced confidence in the efficacy of metformin for cardiovascular disease prevention. Treatment guidelines for major professional associations including the European Association for the Study of Diabetes, the European Society for Cardiology, and the American Diabetes Association, now describe evidence for the cardiovascular benefits of metformin as equivocal. According to the American College of Physicians in 2012, low-quality evidence indicates metformin monotherapy is associated with lower cardiovascular mortality than sulfonylurea monotherapy and metformin monotherapy is associated with fewer cardiovascular events than metformin-sulfonylurea combination therapy. Evidence for other comparisons is described as unclear. A 2014 review found tentative evidence that people treated with sulfonylureas have fewer non-fatal cardiovascular events than those treated with metformin (RR 0.67) but a higher risk of severe low blood sugar events (RR 5.64). There was not enough data available to determine the relative risk of death or of death from heart disease.
Metformin has little or no effect on body weight compared with placebo in type 2 diabetes, although it causes weight loss compared with sulfonylureas since sulfonylureas are associated with weight gain.
There is some limited evidence that metformin may be associated with weight loss in obesity in the absence of diabetes. Metformin has a lower risk of hypoglycemia than the sulfonylureas, although hypoglycemia has uncommonly occurred during intense exercise, calorie deficit, or when used with other agents to lower blood glucose. Metformin modestly reduces LDL and triglyceride levels.