Summary of the principal effects of metformin. The principal antidiabetic effects of metformin occur in the liver, via inhibition of gluconeogenesis, and to a lesser extent in the intestine and muscle, leading to a reduction in hyperglycaemia and blood lipids and to increased insulin sensitivity. These same improvements indirectly procure both cardiovascular protective and anticancer effects. Added to this, metformin directly reduces the risk of cardiovascular disease through actions that affect both macro- and microvascular systems and exerts a direct antitumoural effect on cancer cells. Furthermore, metformin is thought to have a neuroprotective effect in neurodegenerative diseases. The beneficial effects of metformin are similar to those observed with calorie restriction, which help to prolong lifespan. Metformin can therefore be considered a calorie restriction mimetic with possible anti-ageing properties.