Metformin Side Effects and What Helps

Reviewed by the JForH team · 2026-10-30

Metformin's most common side effects are diarrhea, nausea, gas and stomach upset. In the US label trial, 53% had diarrhea versus 12% on placebo, and these effects usually ease within a few weeks. Taking it with food, raising the dose slowly or switching to extended-release often helps. Over years it can lower vitamin B12. Lactic acidosis is very rare and mostly linked to poor kidney function.

What are the most common side effects of metformin?

Stomach and bowel problems. Diarrhea is the most common, followed by nausea, gas, bloating and stomach discomfort.

In the placebo-controlled trial on the US label for regular (immediate-release) metformin, these were the top side effects:

So about half the people on metformin in that trial had some diarrhea. Only 6% stopped the medicine because of it.

The American Diabetes Association (ADA) also lists bloating and stomach discomfort as the main side effects. Metformin rarely causes low blood sugar on its own, and the ADA says it is weight neutral.

How long do metformin side effects last?

For most people, the stomach side effects ease after the first few weeks. The US label says they are common when you start and "generally go away after you take the medicine for a while."

They usually return for a short time when the dose goes up. That's why doctors start low and raise it step by step. The label suggests adding no more than 500 mg a week.

New stomach symptoms that start after months of taking metformin without trouble are different. The label says they are unlikely to be from metformin itself and could be a sign of something more serious, including lactic acidosis. Call your doctor if that happens.

How can I reduce metformin stomach problems?

The ADA names three ways. Ask your doctor which ones fit you:

1. Take it with food. The label says to take metformin with meals.
2. Go up slowly. A smaller starting dose, raised every week or two, gives your gut time to adjust.
3. Ask about the extended-release form. It releases the medicine slowly over the day and is usually taken once a day with the evening meal.

Don't crush, cut or chew extended-release tablets. Swallow them whole.

Is extended-release metformin easier on the stomach?

For many people, yes. In the trial on one US extended-release label, diarrhea affected 9.6% of people on metformin and 2.6% on placebo. Nausea or vomiting affected 6.5% versus 1.5%. Only 0.6% stopped because of diarrhea.

Those numbers come from a different trial than the regular-release figures above, so they aren't a direct comparison. Still, the ADA lists extended-release as one way to ease stomach side effects.

Does metformin cause vitamin B12 deficiency?

It can, over time. Metformin lowers how much B12 your gut absorbs. In controlled trials on the US label, about 7% of people had B12 fall below normal.

The ADA's 2026 Standards of Care say doctors should consider checking B12 from time to time in people on long-term metformin. That matters most if you have anemia or nerve symptoms such as numbness, tingling or burning in the feet. The ADA says the risk is higher at doses of 1,500 mg a day or more, and after 4 to 5 years on the medicine.

In the long-running US Diabetes Prevention Program study, low B12 at 5 years was 4.3% on metformin versus 2.3% on placebo. Each extra year on metformin raised the odds a little more.

Low B12 nerve damage can look like diabetic nerve damage. A simple blood test tells them apart. The label suggests a B12 check every 2 to 3 years.

Is lactic acidosis from metformin common?

No, it's very rare. A Cochrane review pooled studies covering more than 70,000 person-years of metformin use. It found no cases at all. The authors put the highest likely rate at 4.3 cases per 100,000 person-years, similar to people on other diabetes medicines.

The risk goes up when the kidneys can't clear metformin, or with heavy drinking, severe illness, dehydration, or not eating or drinking before surgery. Get emergency care if you feel very weak or unwell, have unusual muscle pain, trouble breathing, or stomach pain with vomiting.

Can I take metformin with kidney disease?

Often, yes, with limits. The rules on the US label use eGFR, a blood test that estimates how well your kidneys filter:

The label says to check eGFR before starting and at least once a year after that.

Can I drink alcohol on metformin?

An occasional drink is not banned, but heavy drinking is risky. The label says alcohol strengthens metformin's effect on lactic acid and warns against drinking too much, whether in one sitting or regularly. Ask your doctor what amount is safe for you.

Do I stop metformin before a CT scan with contrast dye?

Sometimes, and your doctor or the imaging team will tell you. The label says to pause metformin at the time of, or before, a scan with iodinated contrast dye if any of these apply:

• Your eGFR is between 30 and 60.
• You have liver disease, heart failure or a history of heavy drinking.
• The dye is going into an artery, such as for a heart catheter test.

Your kidney function is checked 48 hours later, and you restart if it's stable. The label also says to pause metformin while you can't eat or drink before surgery.

What should I ask my doctor about metformin?

• Would extended-release be easier on my stomach?
• Can we raise my dose more slowly?
• When was my last kidney test, and what is my eGFR?
• Should I have my vitamin B12 checked?
• What should I do with my metformin before a scan or surgery?
• How much alcohol is safe for me?

Don't stop or change your dose on your own. If side effects are bothering you, call your doctor or pharmacist. There's usually a way to make metformin easier to take.

This guide is for education and does not replace advice from your own doctor.

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