Metformin is a widely used drug in the therapy of patients affected by diabetes mellitus. Although some caution is needed in the very old, advanced age per se does not represent a contraindication to metformin use. Despite the fact that its precise mechanism of action it is not completely elucidated, long-term treatment with this drug in monotherapy, improves glycaemic control and reduces cardiovascular mortality in overweight type 2 diabetic patients. Experimental evidence produced over the years suggests that metformin may be useful in some clinical conditions different from diabetes mellitus. In the present review we have examined currently available data about the possible use of metformin as an effective therapeutical agent in pathological conditions different from type 2 diabetes mellitus. On the basis of our investigation, the use of metformin can be suggested in overweigth patients affected by impaired glucose tolerance and/or fasting hyperglycaemia and in subjects affected by polycystic ovary syndrome, while further data are needed in order to prescribe such a drug in patients affected by non-alcoholic steato-hepatitis and in HIV patients on antiretroviral therapy.