Metformin side effects: what's normal and what to do

Reviewed by the JForH team · 2026-10-30

The most common side effects of metformin are feeling sick, diarrhoea, stomach pain and a poor appetite. They affect more than 1 in 10 people, mostly when starting, and usually ease within a couple of weeks. The MHRA says low vitamin B12 may affect up to 1 in 10 people taking it. The dose is limited when kidney function falls, and metformin is stopped below an eGFR of 30. In England, being on metformin qualifies you for free prescriptions with a medical exemption certificate.

What are the most common side effects of metformin?

Tummy problems. The NHS lists feeling or being sick, diarrhoea, stomach pain, loss of appetite and a change in taste. The product information for Glucophage, a common brand, classes these as very common, affecting more than 1 in 10 people. They mostly happen when you first start.

The NHS says most side effects ease after a couple of weeks. If they don't, or they bother you, speak to your pharmacist or GP.

How can I stop metformin giving me diarrhoea?

• Take it with food or straight after a meal, as the NHS advises.
• Start on a low dose and increase slowly. Your GP will usually do this for you.
• Ask about slow-release (modified-release) tablets. Some people find them easier on the stomach.
• Swallow tablets whole with water. Don't crush or chew them.

Don't stop taking it on your own. The NHS warns your blood sugar may rise again if you stop. Your GP can change the dose, the timing or the type of tablet.

Does metformin lower vitamin B12?

Yes, it can. In June 2022 the MHRA, the UK medicines regulator, said low vitamin B12 is a common side effect that may affect up to 1 in 10 people taking metformin. The risk is higher with a bigger dose, longer use, and:

• B12 levels already at the low end of normal
• older age, or conditions that reduce absorption, such as Crohn's disease or a past gastrectomy
• a diet with few B12 sources, such as a strict vegan diet
• long-term omeprazole, lansoprazole or other acid-reducing medicines, or colchicine

The MHRA lists signs to report: extreme tiredness, a sore and red tongue, pins and needles, or pale or yellow skin. Numb feet from low B12 can look like diabetic nerve damage, so ask your GP for a B12 blood test if you notice them. You can keep taking metformin while B12 is put right.

Is metformin safe for my kidneys?

Metformin isn't known to harm healthy kidneys, but your kidneys clear it from the body, so the dose depends on how well they work. Your GP checks this with a blood test called eGFR. The Glucophage product information sets these limits:

Your GP checks your kidney function regularly while you take metformin. If your dose is reduced, that's usually why.

Do I stop metformin before a scan or operation?

Often, yes. The product information says to stop metformin at the time of, or before, a scan using iodinated contrast dye, and at the time of surgery under general, spinal or epidural anaesthetic. It shouldn't be restarted for at least 48 hours, and only once kidney function is stable. The hospital team will tell you what to do.

What if I'm being sick or have diarrhoea?

The NHS says vomiting, diarrhoea or a fever put you at risk of dehydration. Ask a pharmacist or your GP what to do with your metformin while you're ill, or follow the sick day rules your team has given you.

Can metformin cause lactic acidosis?

Very rarely. It's a build-up of acid in the blood. Signs include vomiting, stomach pain, muscle cramps, difficulty breathing and feeling very tired. The NHS says to call 999 if you think you have it.

Can metformin cause a hypo?

Not usually on its own. The NHS doesn't list hypos as a side effect of metformin alone. The risk rises if you also take insulin or a sulfonylurea such as gliclazide. Below 4 mmol/L is a hypo.

Can I drink alcohol on metformin?

Yes, within limits. The NHS says to try to limit alcohol, because heavy drinking raises the risk of serious side effects.

Can I take metformin in pregnancy?

You can, but check with a doctor first, the NHS says. Keeping glucose steady matters more than usual in pregnancy, so tell your GP or diabetes team as soon as you're planning one.

Do I get free prescriptions if I take metformin?

In England, yes, if you apply. People with diabetes can get a medical exemption certificate, except where diabetes is treated by diet alone. Being on metformin counts.

• Ask your GP surgery for the form. The NHS Business Services Authority sends the certificate within 10 working days.
• Certificates for diabetes now last 10 years. Applications received before 16 July 2026 usually got 5 years.
• Without one, each item costs £9.90 in England in 2026/27.
• Prescriptions are free for everyone in Scotland, Wales and Northern Ireland.

If your diabetes is later managed by diet alone, for example in remission, it no longer qualifies in England.

How much does metformin lower HbA1c?

On its own, by about 1.1 percentage points on average compared with a dummy pill, in a 2012 pooled analysis of trials. That's about 12 mmol/mol. So an HbA1c of 62 mmol/mol (7.8%) might fall to around 50 mmol/mol (6.7%), though results vary.

Check how your medicines and numbers fit together (free)

Last updated: 2026-10-30. Educational information, not medical advice. Talk to your doctor before changing your treatment.