Metformin is a generic drug, used for treatment of type 2 diabetes mellitus. Metformin is known to lower the gluconeogenesis in lover cells and improves the insulin signalling in some target cells, thus exhibiting the anti-hyperglycemic activity. At the molecular level the Metformin affects the AMPK pathway and thus the energy level of the cells. Apart from its main action, it is also known to inhibit the activity of complex I of mitochondrial respiratory chain, though with unknown mechanism. This effect of Metformin on the cellular bioenergetics may be associated with its side effects on different tissues mainly skeletal muscle and other associated complaints such as indigestion, lactic acidosis associated heart failure and renal failure etc. Though on one side, the effect of metformin on complex I activity can be associated with the mentioned-above conditions of a diabetic patients but the similar effect of metformin can be harvested for its use as an anticancerous drug. As Metformin is a drug that is suggested to do more good than harm, so this drug has been proposed as a useful agent in a wide variety of metabolic conditions, including cancer. A good number of epidemiological studies suggested a strong protective effect of metformin against a number of cancers by targeting the cellular bioenergetics of tumor cells. The current review focuses upon the importance of metformin as a future anticancerous drug.