Metformin is a biguanide used mainly in the treatment of type 2 diabetes mellitus. It has a number of actions which improves glucose tolerance. Unlike sulphonylureas it does not cause hypoglycaemia and weight gain and is therefore first-line, particularly if the patient is overweight. Metformin is also used in polycystic ovarian syndrome and non-alcoholic fatty liver disease

Mechanism of action

Mechanism of action
  • Acts by activation of the AMP-activated protein kinase (AMPK)
  • Increases insulin sensitivity
  • Decreases hepatic gluconeogenesis
  • May also reduce gastrointestinal absorption of carbohydrates

Adverse effects

  • Gastrointestinal upsets are common (nausea, anorexia, diarrhoea), intolerable in 20%. If patients develop unacceptable side-effects then modified-release metformin should be considered
  • Reduced vitamin B12 absorption - rarely a clinical problem
  • Lactic acidosis with severe liver disease or renal failure


  • Chronic kidney disease: NICE recommend that the dose should be reviewed if the creatinine is > 130 ┬Ámol/l (or eGFR < 45 ml/min) and stopped if the creatinine is > 150 ┬Ámol/l (or eGFR < 30 ml/min)
  • Metformin may cause lactic acidosis if taken during a period where there is tissue hypoxia. Examples include a recent myocardial infarction, sepsis, acute kidney injury and severe dehydration
  • Iodine-containing x-ray contrast media: examples include peripheral arterial angiography, coronary angiography, intravenous pyelography (IVP); there is an increasing risk of provoking renal impairment due to contrast nephropathy; metformin should be discontinued on the day of the procedure and for 48 hours thereafter
  • Alcohol abuse is a relative contraindication