Common indications: 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.
Prescription: 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.
Administration: 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.
Communication: 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.
Monitoring: Assess blood glucose control by measuring glycated haemoglobin (HbA1c metformin) (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.