This review investigates the different biological effect of Metformin (MET) in different conditions. MET is an oral antidiabetic drug used for the treatment of type 2 diabetes mellitus (T2DM) particularly in overweight people. The main mechanism of action of the MET is inhibition of hepatic glucose production and reduction of insulin resistance. In addition to its antidiabetic effects, MET is also found to be related with the risk for development of several human solid cancers types such as colorectal, breast and pancreas cancer in the diabetic patients. Nowadays according to some researches, MET is believed to decrease or prevent aging and mortality. Moreover, clinical and experimental evidence has shown that MET has beneficial effects in patient with obesity, polycystic ovarian syndrome and Alzheimer's disease. Recent studies have shown that activation of adenosine monophosphate-activated protein kinase (AMPK) by MET can explain its beneficial metabolic effects. In this manuscript, a reevaluation of mechanisms as well as pharmacokinetic properties, genetic variants of transporters, drug-drug interactions, side effects and potential clinical benefits of MET have been reviewed.