Metformin
Brand names: Glucophage, Fortamet, Riomet, Diaformin
Biguanide antihyperglycaemic agent
Metformin is the first-line medicine for type 2 diabetes and one of the most studied drugs in medicine. It lowers blood sugar without causing hypoglycaemia and is associated with cardiovascular benefit. Its main drawback is GI intolerance, which can be managed with gradual dose escalation.
Drug Class
Biguanide antihyperglycaemic agent
Category
Diabetes
Forms Available
Tablets (500 mg, 850 mg, 1000 mg), Extended-release tablets (500 mg, 750 mg, 1000 mg)
How It Works
Metformin's primary action is to suppress hepatic glucose production (gluconeogenesis) by activating AMPK, a key energy-sensing enzyme. It also modestly improves peripheral insulin sensitivity and slows intestinal glucose absorption. Unlike sulphonylureas, it does not stimulate insulin secretion and therefore does not cause hypoglycaemia when used alone.
What It's Used For
- Type 2 diabetes mellitus — first-line agent by all major guidelines
- Prevention of type 2 diabetes in high-risk individuals (pre-diabetes)
- Polycystic ovary syndrome (PCOS) — improves insulin resistance and menstrual regularity
- Adjunct to insulin in type 1 diabetes (off-label, reduces insulin requirements)
Available Forms
Side Effects
Common Side Effects
- Nausea, vomiting, diarrhoea, abdominal discomfort — very common at initiation
- Metallic taste in the mouth
- Reduced appetite (can be beneficial in overweight patients)
Serious Side Effects
- Lactic acidosis — rare but potentially fatal; risk greatly increased with severe kidney impairment, liver disease, or acute illness with dehydration
- Vitamin B12 deficiency — with long-term use; annual monitoring recommended
Drug Interactions
- Contrast dye (iodinated) — hold metformin before and 48 hours after IV contrast in at-risk patients (eGFR <60) due to transient kidney impairment risk
- Alcohol — increases lactic acidosis risk
- Cimetidine, trimethoprim — reduce metformin renal excretion
When to Avoid
- eGFR <30 mL/min — avoid; use with caution between 30–45
- Active liver disease or raised transaminases
- Heart failure requiring pharmacological therapy
- Acute illness with risk of dehydration or tissue hypoxia
Patient Tips
- Start low and go slow — begin at 500 mg daily with a meal; increase over weeks to minimise GI side effects
- Extended-release (XR) formulations have fewer GI side effects — ask your doctor about switching
- Take with the largest meal of the day
- Hold metformin if you are hospitalised, having a procedure with contrast dye, or are severely unwell
- Get a B12 level checked annually if you've been on metformin for more than 2 years
Starting metformin: managing the GI side effects
Up to 30% of patients experience significant GI side effects when metformin is started at full doses. The key is to start at 500 mg daily with the main meal and increase by 500 mg every 1–2 weeks to the target dose (usually 1000 mg twice daily).
Extended-release metformin has a significantly lower rate of GI side effects and is often preferable for intolerant patients. It is taken once daily, usually with the evening meal.
Metformin and vitamin B12
Long-term metformin use reduces vitamin B12 absorption in the terminal ileum by interfering with intrinsic factor. Over years, this can cause B12 deficiency and, if severe, peripheral neuropathy or macrocytic anaemia.
Guidelines recommend checking B12 levels every 1–2 years in long-term metformin users. Supplementation is straightforward and inexpensive.
Medical disclaimer
This page is for educational reference only. Always follow the dosage and instructions given by your prescriber or pharmacist. Do not start, stop, or change medicines without medical advice.
Reviewed 2026-06-01 · Looms Medical Team