In type 2 diabetes, insulin produced by the pancreas is not able to get sugar from the blood into the body's energy-hungry cells. Metformin normalizes blood sugar by increasing uptake in the liver, muscle, fat, and other tissues.

The review added more recent trials of metformin since completion of the landmark 1998 United Kingdom Prospective Diabetes Study (UKPDS), the largest and longest diabetes study undertaken to date. It focused on the same health outcomes as the earlier study: diabetes-related problems such as heart attack, stroke, kidney failure, nerve damage, amputation, blindness and death.

"The eight additional trials reporting primary outcomes do not change any of the conclusions of the UKPDS," conclude the review authors. "In overweight type 2 diabetics, intensive metformin shows more benefit than [other drugs] or insulin therapy, for reducing the risk for any clinical end point related to type 2 diabetes, all-cause mortality and stroke."

"Our review confirms that tight glycemic control with metformin monotherapy is one of the main therapeutic options in the type 2 diabetes in patients with overweight or obesity," they say. In addition, they confirm that the adverse effects of metformin, mainly on the digestive tract, are not life-threatening.