Metformin Side Effects: What Helps, According to the Evidence

Metformin has long been the first medicine offered for type 2 diabetes, and it has been used for more than 60 years. The 2026 ADA Standards describe it as a commonly used medicine that has historically been the first-line treatment.^[6] Many people take it with few problems. A good share, though, get gut side effects, mostly in the first weeks.

This article covers the common side effects, what research says about why they happen, and the steps that are backed by the label or by studies. None of it replaces your doctor's advice. Use it to prepare for your next appointment.

This guide is general education. If you use insulin or other glucose medicines, follow the plan your diabetes team gave you.

What are the most common metformin side effects?

Gut symptoms are the most common. In the trial behind the US label, diarrhea was reported by 53% of people on metformin and 12% on placebo, and nausea or vomiting by 26% versus 8%. Gas, indigestion and stomach discomfort were also more common. Diarrhea led 6% of people to stop.^[4] Large observational studies put gut side effects at around 20 to 30% of users.^[1][2]

Some people are more prone than others. Women and older people were more likely to be intolerant in a Scottish study, and so were people with two low-function copies of the OCT1 gene, which moves metformin out of the gut.^[1]

Nausea and taste changes

Nausea is often the first thing people notice. The NHS also lists loss of appetite and a change in the way things taste.^[5] In lab tests on human gut tissue, metformin made the lining release serotonin, a chemical linked to nausea and diarrhea. The researchers suggest this may explain part of the side effects.^[7]

Diarrhea and stomach discomfort

Diarrhea is the most frequent complaint on the label.^[4] Several causes have been proposed, including serotonin release, changes in bile acids and changes in gut bacteria. A review found that none of them fully explains it yet.^[8]

Bloating and gas

Gas was reported by 12% of people on metformin in the label trial, against 6% on placebo.^[4] A randomized study in Nature Medicine showed that metformin strongly changes gut bacteria within four months.^[9]

Keep a short symptom diary for the first weeks. Write down what you ate, when you took the tablet and how you felt afterwards. It gives your doctor something concrete when deciding about dose, timing or formulation. If side effects bother you, talk to your doctor or pharmacist before changing anything.^[5]

Why does metformin cause digestive issues?

A large part of metformin's action seems to happen in the gut itself, which helps explain why the gut takes most of the side effects. A review in Diabetologia describes how metformin raises GLP-1, changes the bile acid pool and alters gut bacteria.^[10]

The gut-first idea

In a 12-week trial, a delayed-release form built to release further down the gut reduced fasting glucose even though only about half as much reached the blood. The authors took this as strong evidence that the main effect starts in the bowel. The study was funded by Elcelyx Therapeutics, the developer of the delayed-release formulation.^[11]

Gut bacteria

A study in Nature that analyzed 784 gut samples found that metformin changes gut bacteria in ways that may help its effect. It also found a rise in Escherichia species, which may be linked to the gut side effects.^[12]

Serotonin

The gut holds much of the body's serotonin, and gut bacteria help control how much is made.^[13] Together with the tissue study above, this is one reason researchers look at serotonin when they study metformin nausea.^[7]

How can you reduce metformin side effects?

The label and the studies point to three practical steps: take it with meals, raise the dose slowly, and ask about the extended-release form. Work through these with your doctor rather than changing anything yourself.

Take metformin with meals

The US label says to take metformin with meals.^[4] In healthy volunteers, food slowed absorption and reduced the amount absorbed by about 24%, with the peak arriving about 37 minutes later.^[14] Slower absorption is one reason food is advised.

Start low, go slow

On the US label, standard metformin usually starts at 500 mg twice a day or 850 mg once a day with meals. The dose then goes up by 500 mg a week or 850 mg every 2 weeks, up to 2,550 mg a day. Doses above 2,000 mg may be easier on the stomach when split over three meals.^[4] In a dose-ranging trial, side effects, mostly gut symptoms, were reported by 28% on metformin and 15% on placebo.^[15]

Ask about extended-release (ER) metformin

ER tablets release the drug more slowly. In a chart review of 205 people switched from standard to ER metformin on similar doses, gut side effects fell from 26% to 12% and diarrhea from 18% to 8%. The review was funded by Bristol-Myers Squibb, maker of Glucophage XR.^[3] A clinical review notes that people switched because of gut problems often tolerate ER better.^[16] These are not results from a randomized trial, so your experience may differ.

Side effects and risks at a glance

Side effect or risk.	How common it is in studies.	What the label or research suggests.
Diarrhea, nausea and stomach upset.	About 20 to 30% in large cohorts, and about 5% stop because of them.[1][2] Diarrhea was reported by 53% in the label trial, and 6% stopped.[4]	Take it with meals and raise the dose slowly.[4] Ask about ER tablets. In one chart review, gut side effects fell from 26% to 12% after switching.[3]
Reduced vitamin B12.	7% dropped below normal over 29 weeks on the label.[4] 4.3% versus 2.3% on placebo at 5 years in the DPP.[18]	The US label advises a blood count every year and a B12 test every 2 to 3 years.[4] The ADA also links B12 deficiency to worse nerve symptoms.[6]
Lactic acidosis.	Very rare. No cases in 70,490 patient-years in a Cochrane review, with an upper limit of 4.3 per 100,000 patient-years.[19]	Not used if eGFR is below 30. Check kidney function at least once a year and tell your care team before contrast scans or surgery.[4]

Does metformin affect vitamin B12 levels?

Yes. In 29-week trials on the US label, about 7% of people whose B12 was normal dropped below normal.^[4] In the Diabetes Prevention Program, B12 deficiency was found in 4.3% of the metformin group and 2.3% of the placebo group after 5 years. Each year of use raised the odds of deficiency.^[18]

Who is most at risk?

Risk grows with how long you take it.^[18] In a Dutch trial of people on insulin taking 850 mg three times a day, B12 deficiency at the end of 4.3 years was 7.2 percentage points more common with metformin than with placebo.^[20]

What to ask your doctor

The US label advises blood counts every year and a B12 test every 2 to 3 years.^[4] The American Diabetes Association also recommends periodic B12 checks during long-term use.^[6] B12 deficiency can cause tiredness, and numbness or tingling in the hands and feet. These overlap with diabetic nerve damage, so a blood test is the way to tell. If your level is under the normal range, your doctor will advise on treatment.

Can diet and lifestyle help with metformin side effects?

Some everyday habits may make the first weeks easier. The evidence for these is weaker than for the steps above, so treat them as things to discuss with your provider.

Smaller meals

Some people find that smaller meals sit better than large ones while their gut adjusts. There is no trial that tests this for metformin, so see how it works for you.

Fiber, added slowly

Fiber is good for gut health, but adding a lot at once can make bloating and gas worse. Add it a little at a time over a few weeks and drink enough water. A large Lancet analysis found that, in the general population, eating 25 to 29 g (0.9 to 1.0 oz) of fiber a day was linked to smaller rates of type 2 diabetes, heart disease and some cancers.^[21] UK guidance is 30 g (1.1 oz) a day for adults.^[22]

Probiotics

Because metformin changes gut bacteria, people often ask about probiotics. There is not yet enough good evidence to recommend them for metformin side effects. Ask your doctor before trying one.

Fluids

Diarrhea makes you lose fluid. If you have it, drink plenty of water or other fluids, and tell your doctor if it does not settle.^[5]

A systematic review found that ginger, at about 1,500 mg a day in divided doses, helped with nausea in clinical trials. Those trials were not in people taking metformin. Ask your pharmacist before taking ginger supplements with your other medicines.^[23]

What about lactic acidosis?

Lactic acidosis is rare but serious, and it carries a boxed warning on the US label.^[4] A Cochrane review pooled 347 trials and cohort studies and found no cases in 70,490 patient-years of metformin use. The estimated upper limit was 4.3 cases per 100,000 patient-years on metformin and 5.4 on other diabetes medicines.^[19]

When the risk goes up

The label lists kidney problems, age 65 or over, scans with contrast dye, surgery, low oxygen states such as acute heart failure, heavy drinking, liver problems and some other medicines as risk factors.^[4] Metformin must not be used if eGFR (a kidney-function score) is below 30, and starting it is not recommended between 30 and 45. It may be paused around contrast scans in some people.^[4]

Warning signs

The label lists feeling unwell, muscle pain, trouble breathing, unusual sleepiness and stomach pain as possible signs.^[4] If you think you have it, the NHS says to call 999 in the UK. In the US, call 911.^[5] Tell your doctor you take metformin before any scan or operation, and have your kidney function checked at least once a year.^[4]

How does metformin compare with other medicines?

In the ADOPT trial, which compared metformin, rosiglitazone and glyburide for about 4 years, metformin caused more gut side effects than rosiglitazone. Rosiglitazone caused more weight gain and swelling, and glyburide caused more low blood sugar. The trial was funded by GlaxoSmithKline, maker of rosiglitazone.^[24] Every medicine has trade-offs, and your doctor weighs them for you.

Why metformin is still widely used

In a dose-ranging trial, metformin reduced HbA1c by 0.6% to 2.0% more than placebo at 500 to 2,000 mg a day. That is about 7 to 22 mmol/mol.^[15] In UKPDS 34, overweight people who started on metformin had a 42% smaller risk of diabetes-related death than those on conventional treatment.^[25] The same study found a higher risk when metformin was added early to a sulfonylurea, which is why results depend on who takes it and how.^[25]

What should you remember about metformin side effects?

Gut side effects are common, especially at the start. Taking metformin with meals, raising the dose slowly and asking about the ER form are the steps with the strongest support. Ask about B12 testing if you take it long term.

1. Take standard metformin with meals, or take extended-release (ER) metformin once daily with your evening meal, swallowing ER tablets whole.^[4][14][17]
2. Start at a small dose and increase gradually over several weeks as guided by your prescriber.^[4][15]
3. Ask your doctor or pharmacist whether switching to extended-release (ER) metformin makes sense if diarrhea or nausea persists.^[3][16]
4. Request a vitamin B12 blood test every 2 to 3 years (or sooner if you have numbness, tingling or fatigue) and an annual kidney function (eGFR) check.^[4][6][18]
5. Tell your care team that you take metformin before any contrast dye scan or surgery, and never stop your tablets without medical advice.^[4][5]

Do not stop the medicine on your own. If side effects bother you or do not go away, your doctor or pharmacist can help.^[5] If food choices are on your mind, our guide to natural and artificial sweeteners is a useful next read.

Metformin on its own rarely causes low blood sugar, but if you take it alongside insulin or a sulfonylurea such as gliclazide or glimepiride, or if nausea makes you skip meals while taking those medicines, your blood glucose can fall too low.^[4][26] Low blood sugar usually means below 4 mmol/L (72 mg/dL) in the UK or below 70 mg/dL (3.9 mmol/L) in the US.^[26][27] Our guide to low blood sugar covers the warning signs. In the UK, the NHS says to call 999 if someone may have very low blood sugar and is not responding normally or is unconscious, and a glucagon injection is not available or you do not know how to use it, or they have not recovered within 10 minutes of a glucagon injection, or they have been drinking alcohol.^[26] In the US, the ADA says to call 911 right away if someone is unconscious and glucagon is not available or no one knows how to use it.^[27]

Questions to ask your diabetes team

• Could switching from standard metformin to extended-release (ER) tablets help with my stomach side effects?^[3][16]
• How should I space my dose increases, and which meal is best for taking my tablet?^[4][17]
• When am I due for my next vitamin B12 test and annual kidney function (eGFR) check?^[4][6][18]
• What should I do with my insulin or sulfonylurea dose if nausea or diarrhea makes it hard to eat normally?^[4][5][26]

Frequently asked questions

Why does metformin cause diarrhea and nausea?

Much of metformin's action happens in the gut itself. Research suggests it changes gut bacteria and bile acids and makes the gut lining release serotonin. Each of these may upset digestion, mostly in the first weeks.^[10][7][12]

Does extended-release (ER) metformin cause fewer side effects?

Often, yes. In a chart review of 205 people switched from standard to ER metformin on similar doses, gut side effects fell from 26% to 12% and diarrhea from 18% to 8%. The review was funded by Bristol-Myers Squibb, maker of Glucophage XR.^[3]

Why should you take metformin with meals?

The US label says to take it with meals. In healthy volunteers, food reduced the amount absorbed by about 24% and delayed the peak by about 37 minutes. Slower absorption is one reason food is advised.^[4][14]

How often should vitamin B12 be checked on metformin?

The US prescribing information advises a blood count every year and a vitamin B12 test every 2 to 3 years during long-term use.^[4]

Where to go next

For more on how diabetes medicines work and what to discuss at your next review, see:

• Ozempic and blood sugar, if your doctor suggests a GLP-1 medicine.
• Insulin 101, for how insulin works and what affects it.
• Is Diabec legit?, for what is in our supplement and what it does not do.

Note: If you are pregnant, breastfeeding, or considering giving any supplement or herbal product to a child, consult your healthcare provider before making any changes to your diet, supplement routine, or prescribed medications.

Disclaimer: These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Talk to your doctor before changing your diet, your activity or your medicines, and before starting any supplement.