Metformin increases peripheral and liver sensitivity to insulin. It therefore reduces the amount of glucose released from the liver and increases the amount taken up and used by the rest of the body. It also reduces the level of fats in blood and helps people stabilise or reduce their weight. Taken together, these help reduce damage to blood vessels throughout the body.

By looking at data contained in 29 separate trials, the Cochrane Review Authors concluded that metformin works well as a single-drug therapy for many people with type 2 diabetes. In addition to helping control diabetes, people who were overweight, had type 2 diabetics and were on intensive metformin were also less likely to be affected by stroke, heart attacks, and are less likely to die.

The authors could not compare metformin with newer drugs, because trials of more recently available drugs such as new sulphonylureas, thiazolidinediones, meglitinides and alpha glucosidase inhibitors did not show any direct comparisons with metformin for these primary outcomes.

They could, however, determine that metformin was as good as, or better than these other drugs at controlling blood glucose and lipids, and body weight.

"Our review of literature suggests that metformin is a good first line therapy for overweight or obese people with type 2 diabetes," says lead author Antonio Saenz, who works as a family physician in Madrid, Spain.