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Type 2 diabetes mellitus, as first choice medication for control of blood glucose, used alone or in combination with other oral hypoglycaemic drugs (e.g. sulphonylureas) or insulin.


Mechanisms of action

Metformin (a biguanide) lowers blood glucose by increasing the response (sensitivity) to insulin. It suppresses hepatic glucose production (glycogenolysis and gluconeogenesis), increases glucose uptake and utilisation by skeletal muscle and suppresses intestinal glucose absorption. It achieves this by diverse intracellular mechanisms, which are incompletely understood. It does not stimulate pancreatic insulin secretion and therefore does not cause hypoglycaemia. It reduces weight gain and can induce weight loss, which can prevent worsening of insulin resistance and slow deterioration of diabetes mellitus.


Adverse effects

Metformin commonly causes gastrointestinal upset, including nausea, vomiting, taste disturbance, anorexia and diarrhoea. This adverse effect may contribute to weight loss in patients taking metformin. Lactic acidosis is a rare adverse effect associated with metformin use, which can be fatal if untreated. It does not occur in stable patients, but can be precipitated by intercurrent illness that causes metformin accumulation (e.g. worsening renal impairment), increased lactate production (e.g. sepsis, hypoxia, cardiac failure) or reduced lactate metabolism (e.g. liver failure).



Metformin is excreted unchanged by the kidney. Metformin is therefore contraindicated in severe renal impairment and a dose reduction is required for patients with moderate renal impairment. Metformin should be withheld acutely where there is acute kidney injury, e.g. in sepsis, shock, or dehydration; or severe tissue hypoxia, e.g. in cardiac or respiratory failure, or myocardial infarction. Caution is required in hepatic impairment as clearance of excess lactate may be impaired. Metformin should be withheld during acute alcohol intoxication, when it may precipitate lactic acidosis, and be used with caution in chronic alcohol overuse, where there is a risk of hypoglycaemia.



Metformin must be withheld before and for 48 hours after injection of IV contrast media (e.g. for CT scans, coronary angiography) when there is an increased risk of renal impairment, metformin accumulation and lactic acidosis. Other drugs (e.g. ACE inhibitors, NSAIDs, diuretics) with potential to impair renal function should also be used with caution (e.g. with renal function monitoring) in combination with metformin. Prednisolone, thiazide and loop diuretics elevate blood glucose, hence oppose the actions and reduce efficacy of metformin.



Metformin is only available for oral administration. Gastrointestinal adverse effects of metformin are usually transient and are best tolerated if metformin is started at a low dose and increased gradually. A common regimen is to start metformin 500 mg once daily with breakfast, increasing the dose by 500 mg weekly to 500–850 mg three times daily with meals. Metformin is a long-term treatment that in general should only be stopped or changed if adverse effects are intolerable or new contraindications develop.



Patients should be started on a standard-release preparation of metformin and advised to swallow tablets whole with a glass of water with or after food. If gastrointestinal effects are intolerable, changing from standard to a modified-release preparation may help.



Advise patients that metformin has been prescribed to control the blood sugar level and reduce the risk of diabetic complications, such as heart attacks. Explain that tablets are not a replacement for lifestyle measures and should be taken in addition to a calorie-controlled diet and regular exercise. Warn them to seek urgent medical advice if they experience vomiting, stomach ache, muscle cramps, difficulty breathing or severe tiredness, which may be symptoms of a very rare side effect called lactic acidosis. Advise them always to tell a doctor that they are taking metformin before having an X-ray or operation, as metformin may need to be stopped before the procedure.



Assess blood glucose control by measuring glycated haemoglobin (HbA1c) (target <58 mmol/mol). Blood glucose monitoring is not routinely required. For safety, measure renal function before starting treatment, then at least annually. Renal function should be measured more frequently (at least twice per year) in people with deteriorating renal function or at increased risk of renal impairment.