The most common metformin side effects are stomach problems: diarrhea, feeling sick, vomiting, tummy pain and loss of appetite. They are most likely in the first weeks, and the NHS says most should ease after a couple of weeks as your body adjusts, although some can last longer. Taking metformin with food, building the dose up slowly and, if needed, switching to a slow-release tablet are the main ways to make it easier on the stomach.

Serious problems are rare, but lactic acidosis, dehydration during illness, kidney problems and some scans or operations all affect how metformin should be used. This guide covers the practical side first, then the safety side.

What metformin does

Metformin is used mainly for type 2 diabetes. The NHS says it is also used for gestational diabetes and for type 1 diabetes alongside insulin, and that it works by lowering your blood sugar level. The US label explains how: metformin decreases the amount of glucose the liver makes, decreases the absorption of glucose from the gut, and improves insulin sensitivity.

In the UK, the NICE guideline NG28 says to offer standard-release metformin as the first-line medicine for adults with type 2 diabetes. For the food side of diabetes care, see our type 2 diabetes food list.

The common stomach side effects, and how often they happen

The NHS lists these common side effects:

feeling sick (nausea)
being sick (vomiting)
diarrhea
tummy pain
loss of appetite
changes to taste
vitamin B12 deficiency (more on this below)

The US prescribing information for Glucophage (metformin) and Glucophage XR (extended-release metformin), revised April 2017, gives figures from clinical trials:

Standard (immediate-release) metformin: diarrhea in 53.2% of people versus 11.7% on placebo; nausea or vomiting in 25.5% versus 8.3%; flatulence (wind) in 12.1% versus 5.5%; indigestion in 7.1% versus 4.1%; and abdominal discomfort in 6.4% versus 4.8%. Diarrhea led 6% of people to stop the study medicine.
Extended-release metformin: diarrhea in 9.6% versus 2.6% on placebo, and nausea or vomiting in 6.5% versus 1.5%. Diarrhea led 0.6% of people to stop.

These come from separate trials, so they are not a head-to-head comparison, and people on placebo had stomach symptoms too.

Why it happens and how long it lasts

The official sources used here do not explain exactly why metformin upsets the stomach, but they do describe the pattern.

The NHS says most side effects should ease after a couple of weeks as your body adjusts, but some can last longer.
The US label's patient information says these stomach side effects generally go away after you take the medicine for a while.
The label also says stomach symptoms are common when metformin is started. Once someone is stable on a dose, new stomach symptoms are unlikely to be caused by the drug, and later stomach symptoms could be due to lactic acidosis or another serious illness.

So if you have been fine for months and then develop vomiting, stomach pain or diarrhea, do not assume it is "just the metformin". Get checked.

How to take metformin to avoid stomach upset

Take it with food. The NHS says most people take it 1 to 3 times a day with food, and the US label says taking it with meals can help reduce stomach side effects.
Let your prescriber build the dose up slowly. NICE recommends gradually increasing the dose of standard-release metformin over several weeks to minimise stomach side effects. The US label agrees. Your prescriber will set the dose and the pace.
Swallow the tablets whole. The NHS says to swallow tablets whole with water, not crushed or chewed. The label says extended-release tablets must be swallowed whole and never crushed or chewed.
Ask about a slow-release tablet if problems continue. NICE says that if an adult has stomach side effects with standard-release metformin, a trial of modified-release metformin should be considered. The American Diabetes Association's (ADA's) 2025 Standards of Care also said stomach side effects can be reduced by gradual dose titration and/or using an extended-release formulation.
Keep up your fluids if you have diarrhea. Diarrhea and vomiting can dehydrate you, which matters with metformin (see the sick day section below).
Talk to your pharmacist or doctor if side effects bother you or do not go away. The NHS says they can suggest ways to reduce them. The US label notes that some people need a lower dose or a break, but that is your prescriber's decision.

Metformin extended-release vs regular

Standard (immediate) release is usually taken with meals, often more than once a day. NICE names it as the first-line choice.
Extended-release, also called modified-release or slow-release (MR, XR or SR), releases the medicine gradually. The US label says Glucophage XR should generally be given once daily with the evening meal. In trials it had lower rates of diarrhea than the standard tablets had in their own trials (see the figures above).

The label notes that the inactive ingredients of the extended-release tablet occasionally pass out in the stool as a soft mass.

Do not swap between the two types yourself. Any change needs a new prescription and instructions from your prescriber.

Taste changes

The US label's patient information says about 3 in every 100 people taking Glucophage or Glucophage XR notice an unpleasant metallic taste, and describes it as short-lived. Mention it if it does not pass.

Low blood sugar

The US label says low blood sugar (hypoglycemia) does not occur with metformin alone under usual circumstances.

The risk changes when metformin is combined with other medicines. The label says low blood sugar is more likely when metformin is taken with a sulfonylurea (such as gliclazide or glimepiride) or with insulin. Its patient information adds that low blood sugar can happen if you do not eat enough, if you drink alcohol, or if you take other medicines to lower blood sugar.

If you take insulin or a sulfonylurea as well, ask your prescriber for a low blood sugar plan. If you also use a GLP-1 medicine, our guide to GLP-1 side effects covers its stomach effects, which can add to metformin's.

Lactic acidosis: rare but serious

Lactic acidosis means too much acid builds up in the blood. The US label carries a boxed warning about it and calls it a rare but serious condition. The ADA's 2025 Standards of Care described it as very rare. It is a medical emergency that must be treated in hospital.

The label's risk factors are kidney problems, certain other medicines, age 65 or over, scans with contrast dye, surgery and other procedures, low-oxygen states such as acute heart failure, excessive alcohol and liver problems. The patient leaflet adds dehydration (from fever, vomiting, diarrhea or heavy sweating), a heart attack, a severe infection and a stroke.

Kidney checks

Kidney function affects whether, and how, metformin can be used safely. It is measured with a blood test called eGFR. The US label says:

check eGFR at least once a year in everyone taking metformin
metformin must not be used (is contraindicated) if eGFR is below 30 mL/min/1.73 m2
starting metformin is not recommended if eGFR is between 30 and 45 mL/min/1.73 m2
if eGFR falls below 45 mL/min/1.73 m2 in someone already taking metformin, the prescriber should weigh up the benefits and risks of continuing
stop metformin if eGFR later falls below 30 mL/min/1.73 m2

Go for your blood tests, and ask what your result means for you.

Contrast scans and surgery

Some X-ray and CT scans use an iodine-based dye. The US label says to stop metformin at the time of, or before, an iodinated contrast imaging procedure in people with an eGFR between 30 and 60 mL/min/1.73 m2, people with a history of liver disease, alcoholism or heart failure, and people who will be given contrast into an artery. It says to re-check eGFR 48 hours after the scan, and to restart metformin if kidney function is stable.

For surgery, the label says metformin should be stopped for a time while food and fluid intake are restricted.

Tell the team doing your scan or operation that you take metformin, and follow their instructions on stopping and restarting.

Sick day rules: vomiting, diarrhea and fever

Illness that dehydrates you raises the risk with metformin. The NHS says that if you have vomiting, diarrhea or a fever, you may become dehydrated, and to speak to a pharmacist or doctor, or follow advice you have already been given.

Scotland's medicines sick day guidance card (Version 4, 2026, from the Scottish Government and Healthcare Improvement Scotland) gives clear wording. It applies when you have an illness with:

two or more episodes of vomiting or diarrhea, or
fever, sweats or shaking (unless mild)

During that illness, it lists metformin among the medicines to stop temporarily, as previously agreed with your healthcare professional. Others on the card include SGLT-2 inhibitors, ACE inhibitors, ARBs, diuretics and anti-inflammatory painkillers such as ibuprofen. It says to restart once you are well, usually after 24 to 48 hours of eating and drinking normally.

The key phrase is "as previously agreed with your healthcare professional". Ask your doctor or pharmacist now, while you are well, whether these rules apply to you. If you take insulin, ask for a sick day plan that covers it too.

Alcohol

The NHS says to try to limit alcohol while taking metformin, because it can increase the risk of serious side effects. The US label says alcohol makes metformin's effect on lactic acid stronger, and its patient information says not to drink a lot of alcohol, as it can increase the chance of lactic acidosis.

Alcohol can also contribute to low blood sugar, as noted above. Ask your pharmacist what is sensible for you. If you also use semaglutide or tirzepatide, see alcohol on Ozempic or Mounjaro.

Vitamin B12

Long-term metformin can lower vitamin B12 levels. The UK regulator, the MHRA, says vitamin B12 deficiency is a common side effect that may affect up to 1 in 10 people taking metformin, and advises seeking medical advice for new or worsening extreme tiredness, which can be a symptom. You can usually keep taking metformin while low B12 is corrected. For symptoms, who should be tested and treatment, see our full guide to metformin and vitamin B12 deficiency.

If you miss a dose

The NHS advice is:

If you take it once a day: take it as soon as you remember, unless it is nearly time for your next dose.
If you take it more than once a day: if your next dose is nearly due, skip the missed one and take the next dose at the usual time.
Never take two doses to make up for a missed one.

If you take extra by mistake, the NHS says an extra dose can be dangerous and to get urgent advice (it names NHS 111 in the UK; elsewhere, contact your doctor or local urgent care). The US label notes that lactic acidosis was reported in about 32% of metformin overdose cases.

Why not to stop without advice

The NHS advises keeping on with metformin unless you are told to stop, because your blood sugar may rise again if you stop. A slower increase, a slow-release tablet or another medicine can be discussed instead. If your readings are running high, for example first thing in the morning, see high fasting blood sugar in the morning and take your results to your next review.

Questions to ask your doctor or pharmacist

Am I on standard-release or slow-release metformin, and when should I take it?
My stomach has not settled. Can the dose increase be slowed, or can I try a modified-release tablet?
What was my last eGFR, and when is my next kidney test?
Should I follow sick day rules, and which of my medicines do they cover?
I have a scan or operation coming up. When should I stop and restart metformin?
Should my vitamin B12 be checked?

Frequently asked questions

How long does metformin diarrhea last?

The NHS says most side effects should ease after a couple of weeks, but some can last longer. If diarrhea does not settle, or starts after months on a stable dose, see your doctor, as the US label warns later symptoms can signal lactic acidosis or another illness.

Should I take metformin before or after food?

With food. The NHS and the US label both say to take it with food or meals, which helps reduce stomach side effects. Follow the timing on your own prescription.

Is extended-release metformin better for the stomach?

It often is. It had lower diarrhea rates in the US label's trials, and NICE says a trial of modified-release metformin should be considered if standard-release causes stomach side effects.

Can I drink alcohol while taking metformin?

Limit it. The NHS says alcohol can increase the risk of serious side effects, and the US label links heavy drinking with lactic acidosis.

Should I stop metformin if I have a stomach bug?

Scottish sick day guidance says metformin may need pausing during an illness with two or more episodes of vomiting or diarrhea, or with fever, sweats or shaking (unless mild). It says to restart once you are well, usually after 24 to 48 hours of eating and drinking normally. Agree this with your doctor or pharmacist in advance, and get help if you cannot keep fluids down.

If you'd like a fuller guide, my book The Type 2 Diabetes Food List and Meal Plan explains what to eat and how common diabetes medicines work.

This article is general information and does not replace advice from your own doctor or pharmacist.

Sources

NHS. Metformin (page last reviewed 5 August 2026)
NHS. Side effects of metformin (reviewed 5 August 2026)
NHS. How and when to take metformin (reviewed 5 August 2026)
NHS. Common questions about metformin (reviewed 5 August 2026)
U.S. Food and Drug Administration. GLUCOPHAGE and GLUCOPHAGE XR (metformin) prescribing information, revised April 2017
National Institute for Health and Care Excellence. Type 2 diabetes in adults: management (NG28), recommendations 1.7.3, 1.7.7 and 1.7.8
American Diabetes Association. 9. Pharmacologic approaches to glycemic treatment: Standards of Care in Diabetes 2025. Diabetes Care. 2025;48(Suppl 1):S181-S206.
Medicines and Healthcare products Regulatory Agency. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk (Drug Safety Update, 20 June 2022)
Scottish Government and Healthcare Improvement Scotland. Medicines sick day guidance card (Version 4, 2026)