Below is a transcript of the latest statement (December 2008) published on Diabetes Care about the role of metformin in the medical management of hyperglycemia in type 2 diabetes.
Metformin
In most of the world, metformin is the only biguanide available. Its major effect is to decrease hepatic glucose output and lower fasting glycemia. Typically, metformin monotherapy will lower A1C levels by ~ 1.5 percentage points (27,49). It is generally well tolerated, with the most common adverse effects being gastrointestinal. Metformin monotherapy is not usually accompanied by hypoglycemia and has been used safely, without causing hypoglycemia, in patients with prediabetic hyperglycemia (50). Metformin interferes with vitamin B12 absorption but is very rarely associated with anemia (27).
The major nonglycemic effect of metformin is either weight stability or modest weight loss, in contrast with many of the other blood glucose lowering medications. The UKPDS demonstrated a beneficial effect of metformin therapy on CVD outcomes (7), which needs to be confirmed. Renal dysfunction is considered a contraindication to metformin use because it may increase the risk of lactic acidosis, an extremely rare (less than 1 case per 100,000 treated patients) but potentially fatal complication (51). However, recent studies have suggested that metformin is safe unless the estimated glomerular filtration rate falls to < 30 ml/min (52).