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Antidiabetic / biguanide

Metformin: uses, side effects & precautions

Also known as Glucophage, Glucophage XR, Fortamet, Glumetza, Riomet, Diaformin, Metsol.

This information is for general guidance only and does not constitute medical advice. Always consult a qualified healthcare professional for personal medical concerns. In an emergency, call your local emergency number.

What it is used for

  • Type 2 diabetes (first-line blood-sugar control)
  • Prediabetes (in some cases, on a doctor's advice)
  • Polycystic ovary syndrome (PCOS) to help with insulin resistance and ovulation

How it works

Metformin lowers blood sugar mainly by reducing the amount of glucose the liver releases and by helping the body's tissues respond better to its own insulin. It does not usually cause the blood sugar to drop too low on its own.

How it is taken

Comes as standard (immediate-release) tablets, slow-release (extended-release) tablets, and a liquid. It is usually taken by mouth with or just after food to reduce stomach upset. Take it exactly as your doctor, pharmacist, or the label directs, and do not change how much you take without medical advice.

No doses here on purpose. The right strength and schedule depend on you — only a doctor or pharmacist should set them. Always follow your prescription and the label.

Common side effects

  • Nausea or upset stomach
  • Diarrhoea
  • Stomach cramps or bloating
  • Loss of appetite
  • Metallic taste in the mouth
  • Reduced vitamin B12 over long-term use

Serious effects — get help

  • Lactic acidosis (a rare but serious build-up of acid) — signs include unusual muscle pain, trouble breathing, severe drowsiness, feeling very cold, or a fast/irregular heartbeat; seek emergency care immediately
  • Signs of a severe allergic reaction such as rash, swelling, or difficulty breathing

If a reaction is severe — for example trouble breathing, swelling of the face or throat, or collapse — go to the nearest emergency room now or call your local emergency number. Do not wait for an online consultation.

Interactions to watch

  • Iodine-containing contrast dye used for some X-ray/CT scans (may need to be paused around the scan)
  • Other diabetes medicines and insulin (added risk of low blood sugar)
  • Alcohol, especially heavy drinking (raises the risk of lactic acidosis)
  • Some water tablets (diuretics) and medicines affecting the kidneys

Who should take care

  • Tell your doctor if you have kidney or liver problems, as the dose may need adjusting or the drug avoided
  • Use with caution if you have heart failure or a serious infection, dehydration, or are having surgery
  • Discuss pregnancy and breastfeeding with your clinician — metformin is sometimes used in pregnancy but only under medical supervision
  • Long-term users may need periodic checks of kidney function and vitamin B12

Frequently asked questions

Does metformin cause low blood sugar?
On its own metformin rarely causes blood sugar to drop too low. The risk goes up if it is combined with insulin or certain other diabetes medicines, or if you skip meals or drink alcohol.
Why does metformin upset my stomach?
Stomach side effects are common when starting, and taking it with food can help. They often ease over a few weeks. If they persist, ask your doctor about the slow-release form.
Can I drink alcohol while taking metformin?
Occasional light drinking is usually acceptable, but heavy or binge drinking raises the risk of a rare serious condition called lactic acidosis. Discuss your situation with your doctor or pharmacist.
Do I need any regular tests?
Yes — your clinician will usually check your kidney function periodically, and may check vitamin B12 levels with long-term use.
Is Metformin safe to take?
For most people it is safe when taken exactly as a doctor or pharmacist advises and as printed on the label. Tell your clinician about your other medicines and health conditions, and seek urgent care for any serious reaction.

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