Metformin for type 2 diabetes is the first-line medication recommended by NICE for adults newly diagnosed with this condition. As a biguanide drug, metformin works primarily by reducing glucose production in the liver and improving how the body uses insulin, without causing weight gain or significant risk of low blood sugar when used alone. With over 60 years of safe use, metformin remains the cornerstone of type 2 diabetes management in the UK, offering proven effectiveness, a favourable safety profile, and low cost. This article explains how metformin works, who can take it, dosing guidance, and how to manage common side effects.

What Is Metformin and How Does It Work for Type 2 Diabetes?

Metformin is a biguanide medication and the first-line pharmacological treatment for type 2 diabetes mellitus in the United Kingdom, as recommended by the National Institute for Health and Care Excellence (NICE). It has been used safely and effectively for over six decades and remains the cornerstone of diabetes management due to its proven efficacy, favourable safety profile, and low cost.

The primary mechanism of action of metformin involves reducing hepatic glucose production, particularly by decreasing gluconeogenesis in the liver. This helps lower fasting blood glucose levels, which are typically elevated in people with type 2 diabetes. Additionally, metformin enhances insulin sensitivity in peripheral tissues, particularly skeletal muscle, allowing cells to utilise glucose more effectively. Unlike some other diabetes medications, metformin does not stimulate insulin secretion from the pancreas, which means it carries a very low risk of causing hypoglycaemia (dangerously low blood sugar) when used alone. However, the risk of hypoglycaemia increases when metformin is combined with insulin or sulfonylureas.

Metformin also offers several metabolic benefits beyond glucose control. It has a modest effect on weight, often resulting in weight neutrality or slight weight loss, which is particularly beneficial for many patients with type 2 diabetes who are overweight or obese. Some evidence suggests metformin may improve lipid profiles, and whilst the UKPDS (UK Prospective Diabetes Study) indicated possible cardiovascular benefits, these remain uncertain and are the subject of ongoing research. The medication is available in both immediate-release and modified-release (prolonged-release) formulations, allowing flexibility in dosing schedules to suit individual patient needs and tolerability.

Who Can Take Metformin for Type 2 Diabetes in the UK?

NICE guidelines recommend metformin as the initial drug treatment for adults with type 2 diabetes, typically offered at or soon after diagnosis alongside lifestyle modifications—including dietary changes, weight management, and increased physical activity. Metformin is particularly recommended for adults with a body mass index (BMI) of 25 kg/m² or above (lower thresholds apply for people from South Asian and other minority ethnic groups). It is suitable for most adults with type 2 diabetes, including those who are overweight, as it does not promote weight gain and may support modest weight reduction.

However, metformin is not appropriate for everyone. Key contraindications include:

• Severe renal impairment: Metformin is contraindicated when the estimated glomerular filtration rate (eGFR) falls below 30 mL/min/1.73 m². Metformin should not be initiated if eGFR is between 30–44 mL/min/1.73 m². If already taking metformin and eGFR falls to 30–44 mL/min/1.73 m², the dose must be reduced and renal function monitored closely (typically every 3–6 months). Careful monitoring is also needed for those with eGFR 45–60 mL/min/1.73 m².

• Acute metabolic acidosis: Including diabetic ketoacidosis or lactic acidosis.

• Severe hepatic impairment: Due to increased risk of lactic acidosis.

• Conditions predisposing to tissue hypoxia: Such as acute heart failure, recent myocardial infarction, severe respiratory failure, sepsis, or dehydration.

• Excessive alcohol intake: Heavy or binge drinking increases the risk of lactic acidosis.

Metformin should be temporarily discontinued at or just before procedures involving iodinated contrast media and before major surgery (particularly under general, spinal, or epidural anaesthesia). It should be restarted no earlier than 48 hours afterwards, once renal function has been confirmed as stable and normal oral intake has resumed. During acute illness causing dehydration, vomiting, diarrhoea, or sepsis, metformin should be stopped temporarily ('sick-day rules') and restarted only when the patient is well and eating and drinking normally.

Women who are pregnant or planning pregnancy may continue metformin under specialist guidance, as it is increasingly used in the management of diabetes in pregnancy. This should always be under the care of a specialist diabetes team, in line with NICE guidance on diabetes in pregnancy. Regular monitoring of renal function is essential, typically annually in stable patients, but more frequently (every 3–6 months) in those with declining kidney function or other risk factors. Healthcare professionals will assess individual suitability based on medical history, current health status, and laboratory results before prescribing metformin.

How to Take Metformin: Dosage and Administration

Metformin therapy typically begins with a low dose to minimise gastrointestinal side effects, with gradual titration upwards based on glycaemic response and tolerability. The standard starting dose is 500 mg once daily, taken with or after the evening meal. After 1–2 weeks, if tolerated, the dose may be increased to 500 mg twice daily (with breakfast and evening meal).

Further dose escalation continues at weekly or fortnightly intervals, depending on blood glucose monitoring results and side effect profile. A typical target dose is 1,000 mg twice daily (2,000 mg total daily dose) if tolerated. The maximum licensed dose for immediate-release metformin in the UK is 3,000 mg daily, divided into two or three doses. Taking metformin with or immediately after meals significantly reduces gastrointestinal discomfort.

For patients with reduced kidney function (eGFR 30–44 mL/min/1.73 m²), lower maximum doses apply and closer monitoring is required. Metformin should not be initiated in this eGFR range; if already established, dose reduction and regular renal function checks (every 3–6 months) are essential.

Modified-release metformin is an alternative formulation that may improve tolerability for patients experiencing persistent gastrointestinal side effects with standard tablets. It is typically taken once daily with the evening meal, starting at 500 mg and titrated up to a maximum of 2,000 mg daily (some modified-release brands may allow split dosing if needed for tolerability—follow the specific product information). The modified-release preparation provides more gradual drug absorption, which can reduce side effects whilst maintaining glycaemic control.

Important administration advice:

• Swallow tablets whole with water; do not crush or chew, especially modified-release formulations.

• Maintain consistent timing with meals to optimise absorption and minimise side effects.

• If a dose is missed, take it as soon as remembered with food, unless it is nearly time for the next dose—never double up.

• Continue taking metformin even when feeling well, as type 2 diabetes requires ongoing management.

Regular HbA1c monitoring (typically every 3–6 months) helps assess treatment effectiveness, and dose adjustments should always be made in consultation with a healthcare professional.

Common Side Effects and How to Manage Them

Gastrointestinal side effects are the most frequently reported adverse reactions to metformin, affecting up to 20–30% of patients, particularly during initial treatment. Common side effects include:

• Nausea and vomiting

• Diarrhoea

• Abdominal pain or discomfort

• Loss of appetite

• Metallic taste in the mouth

These symptoms are usually mild to moderate, transient, and often resolve within a few weeks as the body adjusts to the medication. Taking metformin with or immediately after meals, starting with a low dose, and increasing gradually can significantly reduce these effects. Switching to modified-release metformin is an effective strategy for patients with persistent gastrointestinal intolerance.

Vitamin B12 deficiency is a common effect of long-term metformin use. Metformin can reduce vitamin B12 levels, and the MHRA (Medicines and Healthcare products Regulatory Agency) advises that healthcare professionals should monitor vitamin B12 levels if symptoms or signs of deficiency develop, or in patients at risk (for example, those with anaemia or peripheral neuropathy). Symptoms of B12 deficiency include fatigue, peripheral neuropathy (tingling or numbness in hands and feet), and anaemia. Periodic monitoring may be considered in at-risk patients, and supplementation should be given when deficiency is confirmed.

Lactic acidosis is a very rare but serious adverse effect. Risk is highest in those with renal impairment, hepatic disease, conditions causing tissue hypoxia, excessive alcohol intake, or acute dehydration. Symptoms include rapid or difficult breathing, severe fatigue or drowsiness, severe abdominal pain, unusual muscle pain or cramps, and feeling very unwell. If you experience these symptoms, stop taking metformin immediately and seek urgent medical attention or call 999 if severely unwell. This constitutes a medical emergency.

Sick-day rules and alcohol: During acute illness with vomiting, diarrhoea, dehydration, or sepsis, stop metformin temporarily and restart only when you are well and eating and drinking normally. Limit alcohol intake and avoid binge drinking, as excessive alcohol increases the risk of lactic acidosis.

When to contact your GP or healthcare team:

• Persistent or severe gastrointestinal symptoms lasting beyond 2–3 weeks

• Signs of vitamin B12 deficiency (unexplained fatigue, neurological symptoms)

• Symptoms suggestive of lactic acidosis (stop metformin and seek immediate help)

• Difficulty maintaining adequate nutrition or hydration due to side effects

Reporting side effects: If you experience any side effects, talk to your doctor, pharmacist, or nurse. You can also report side effects directly via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk or by downloading the Yellow Card app. Reporting helps provide ongoing safety information about this medicine.

Most patients tolerate metformin well with appropriate dose titration and administration guidance, making it a safe and effective long-term treatment for type 2 diabetes.

Frequently Asked Questions