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Why Does Metformin Upset Your Stomach, and Does It Get Better Over Time?

Stomach upset is a common side effect of metformin. Diarrhea, nausea, gas, bloating, and a metallic taste are most noticeable when you start the medicine or raise the dose, and they are more likely with immediate-release tablets or a fast dose increase.

Why Does Metformin Upset Your Stomach, and Does It Get Better Over Time?
Medical Conditionsdiabetes medication side effectsmedication-info
Written By: DocAi Health Editorial Team
Last Updated: 2026-09-30
Medical Disclaimer: This article is for general informational and educational purposes only and is not medical advice. It does not create a doctor-patient relationship and is not a substitute for professional diagnosis or treatment by a qualified healthcare provider. Never disregard or delay seeking professional medical advice because of something you have read here. If you think you may have a medical emergency, call 911 or your local emergency number right away. Health information can change and this guidance is general and US-focused, so consult a licensed clinician in your own country about your specific situation.
Stomach upset is a common side effect of metformin. Diarrhea, nausea, gas, bloating, and a metallic taste are most noticeable when you start the medicine or raise the dose, and they are more likely with immediate-release tablets or a fast dose increase. Several effects in the gut are thought to contribute, and the exact reason some people react more than others is not fully understood. Many people improve over the following weeks, and taking metformin with meals, raising the dose gradually, or (with your prescriber's guidance) switching to the extended-release form can help.

Why Metformin Can Upset Your Digestive Tract

Only part of each metformin dose reaches your bloodstream. The FDA labeling gives its absolute bioavailability as about 50% to 60%, so a sizeable share passes through your intestines without being absorbed. Stomach and bowel symptoms are thought to result from several effects in the digestive tract, including changes in gut signaling, bile acid handling, glucose handling in the gut, and the gut microbiome. Together these may loosen stools and cause gas, bloating, cramping, and nausea. Why some people react more than others is not fully understood.
A 2026 narrative review describes these symptoms as an interaction between exposure and susceptibility. Exposure factors include the dose, the size of each dose, how fast the dose is raised, the formulation, whether you take it with meals, kidney function, and other medications. Individual factors include baseline digestive sensitivity, the gut microbiome, and how your body handles bile acids.
These symptoms are a side effect of the drug rather than an allergic reaction, which more typically causes a rash, swelling, or hives. For many people they improve with time or with a change in dose, timing, or formulation, and if they do not, your prescriber has several adjustments to try.

What to Expect in the First Few Weeks

Prescribers usually start metformin at a low dose and raise it gradually. MedlinePlus notes that increases are made no more often than once every 1 to 2 weeks, and the American Diabetes Association's 2026 Standards of Care note that digestive side effects can be mitigated by gradual dose titration or by using an extended-release formulation. Symptoms are often most noticeable when treatment starts or the dose goes up, and many people improve over the following weeks. If symptoms return after a dose increase, that can be part of the same adjustment. If they return without a dose change, or start later in treatment, tell your prescriber.
The FDA patient information says the common side effects, which include diarrhea, nausea, and upset stomach, generally go away after you take the medicine for a while, and that taking metformin with meals can help lessen them. It advises telling your doctor if the side effects bother you a lot, last for more than a few weeks, come back after they have gone away, or start later in therapy. The American Diabetes Association's patient page on oral diabetes medications likewise says the diarrhea sometimes caused by metformin is improved when it is taken with food (see also our guide on whether metformin has to be taken with food). Skipping doses on your own to avoid discomfort is not recommended. Tell your prescriber instead, so they can adjust the plan.

Who Notices Metformin Stomach Upset the Most

Not everyone reacts to metformin the same way. Gradual dose titration is a standard way to limit digestive side effects, and immediate-release tablets are more likely than extended-release tablets to cause bloating and diarrhea. Together with the individual differences in the gut described above, that is part of why two people on the same dose can have very different experiences.
If you already have a sensitive digestive system, for example irritable bowel syndrome or frequent stomach upset, tell your prescriber. Baseline digestive sensitivity is one of the individual factors the review lists. A slower dose increase, the extended-release form, or both may be worth discussing before you assume metformin will not work for you.

Does Metformin Stomach Upset Get Better Over Time?

For many people, yes, although how long it takes varies from person to person. The FDA patient information says these side effects generally go away after you take the medicine for a while. It also says about 3 in 100 people notice an unpleasant metallic taste when they start metformin, and that the taste lasts for a short time.
Some people notice reduced appetite or modest weight loss on metformin. A small amount can be welcome for people managing type 2 diabetes, but weight loss that is rapid, keeps going, or is unexpected is worth mentioning to your prescriber (see our guide to the causes of unexplained weight loss).
Some people keep having loose stools, gas, or stomach discomfort even after the dose is stable. If that describes you, raise it with your prescriber rather than assuming you have to live with it or quietly stopping the medication on your own. The 2026 review encourages clinicians to address modifiable factors first and to avoid stopping metformin too early when possible.

Immediate-Release vs Extended-Release Metformin

Metformin comes in two main forms. Immediate-release tablets release the full dose fairly quickly and are usually taken with meals two or three times a day. Extended-release metformin, often labeled ER or XR, is designed to release the medication more slowly. MedlinePlus notes that it is usually taken once daily with the evening meal and must be swallowed whole, not split, chewed, or crushed.
Digestive side effects were reported less often with extended-release tablets in the trials described in the FDA labeling: 10% of people taking Glucophage XR reported diarrhea compared with 3% on placebo, while 53% of people taking immediate-release Glucophage reported it compared with 12% on placebo. Those were separate trials, so the numbers are not a direct comparison. A 2022 systematic review of 71 randomized trials concluded that bloating and diarrhea were more likely with immediate-release than with extended-release metformin, and the 2026 review notes that switching to extended-release may reduce symptom burden in some patients.
Some people who have trouble with immediate-release metformin do better on the extended-release version, and prescribers often consider that switch when taking it with meals or raising the dose more slowly has not been enough. Make that change with your prescriber rather than on your own, because the two forms are dosed differently. With the extended-release form, the FDA patient information says you may sometimes pass a soft mass in your stool that looks like the tablet, which is not harmful.

Practical Ways to Ease Metformin Side Effects

A few habits may help many people:
  • Take metformin with meals as directed on your prescription label. If you take an extended-release formulation, follow the specific instructions for that product.
  • Follow your prescriber's titration schedule instead of raising the dose faster than directed.
  • Ask your prescriber how much alcohol, if any, is safe for you on metformin. Alcohol can worsen digestive symptoms in some people, and heavy or binge drinking raises the risk of lactic acidosis (see our guide on drinking alcohol while taking metformin).
  • If you have diarrhea, drink fluids regularly through the day, and get medical care right away if you cannot keep fluids down (see the emergency guidance below).
  • Ask your prescriber whether changing the timing or dividing doses is appropriate for your specific metformin formulation. Extended-release tablets should be swallowed whole, so do not split them yourself.
  • Ask whether extended-release metformin makes sense for you if these changes are not enough.
Do not stop metformin on your own because of stomach upset. Your diabetes still needs to be managed while your gut adjusts, and stopping on your own can leave your blood sugar less well controlled. Call your prescriber instead. They can lower the dose, slow the titration schedule, switch you to extended-release metformin, or choose a different medication if metformin is not a good fit for you.

Metformin, B12, and Effects That Are Not the Same as Stomach Upset

Metformin is also associated with a separate, longer-term effect on vitamin B12, unrelated to the early diarrhea and nausea. The FDA labeling reports that B12 fell below normal in about 7% of people in clinical trials lasting 29 weeks, possibly because metformin interferes with B12 absorption, and it advises periodic monitoring. The American Diabetes Association's 2026 Standards of Care note that metformin use is associated with an increased risk of B12 deficiency and worsening of neuropathy symptoms, and suggest periodic testing of B12 levels. Low B12 usually develops gradually and can cause fatigue and tingling or numbness in the hands or feet, long after any initial stomach upset has resolved. Ask your prescriber whether and how often a B12 check makes sense for you (see our guide to borderline low vitamin B12 results).
Metformin rarely causes low blood sugar by itself, according to the FDA patient information, and NIDDK explains that it helps your liver make less glucose and helps your body use insulin better. Low blood sugar can still happen if you take metformin with insulin or a sulfonylurea, drink alcohol, or do not eat enough. Shakiness, sweating, or confusion in those situations may point to low blood sugar rather than a metformin stomach side effect (see the signs your blood sugar is dropping too low). If you are newer to a type 2 diabetes diagnosis, our overview of type 2 diabetes causes, symptoms, and management covers the bigger picture.

When Stomach Upset Might Signal Something More Serious

Ordinary metformin-related nausea, gas, and diarrhea are usually unpleasant rather than dangerous. Two situations can turn that discomfort into something that needs prompt attention.
The first is dehydration. If diarrhea or vomiting is severe or you cannot keep fluids down, you can lose enough water and salts to strain your kidneys (see the signs of dehydration), and because your kidneys clear metformin from your body, reduced kidney function can let the drug build up. The FDA patient information lists dehydration from fever, vomiting, or diarrhea as something that raises the chance of lactic acidosis.
MedlinePlus advises telling your prescriber if you have severe vomiting, diarrhea, or fever, or are drinking much less than usual, because you may need to pause metformin until you recover. That is a decision for your prescriber, not something to do on your own.
Kidney function matters for metformin safety in general. The FDA labeling advises assessing kidney function (eGFR) before starting metformin and periodically afterward, and it says metformin should not be used when the eGFR is below 30 mL/minute/1.73 m2. Ask your prescriber how often your kidney function is checked (see our guide to low eGFR results).
The second is lactic acidosis, a rare but serious complication in which lactic acid builds up in the blood faster than the body can clear it. The FDA labeling lists reduced kidney function, age 65 or older, liver disease, heavy alcohol intake, conditions that lower oxygen levels such as acute heart failure, surgery, some other medicines, and imaging tests that use iodinated contrast dye among the risk factors. Early symptoms can be vague: the labeling describes them as nonspecific, such as malaise, muscle pain, breathing difficulty, unusual sleepiness, and stomach pain, with low body temperature, low blood pressure, and a slow or irregular heartbeat in severe cases. Because these symptoms overlap with many other problems, you cannot diagnose lactic acidosis from a checklist.
Loose stools on their own are not the warning sign. If you feel seriously unwell in the ways described here while taking metformin, get medical care right away, as listed in the emergency section below, and tell the team you take it.
Some people may need to temporarily stop metformin around an iodinated contrast imaging study, depending on kidney function, the type of contrast procedure, and other risk factors. The FDA labeling advises rechecking kidney function about 48 hours after the procedure before restarting. Tell the imaging team that you take metformin, and follow the instructions from your clinician or the imaging team rather than stopping or restarting it on your own.

When to Seek Medical Care

When to Seek Urgent or Emergency Care

Metformin-related stomach symptoms are usually uncomfortable rather than dangerous, but a small number of presentations need immediate medical attention because they can signal a rare complication called lactic acidosis or severe dehydration affecting your kidneys. If you go to the emergency room, tell the team that you take metformin. This guidance is in addition to, not a replacement for, the general disclaimer above.

Emergency, call 911 or go to the emergency room immediately if:

  • Feeling seriously unwell with unusual muscle pain, unusual sleepiness, or extreme tiredness or weakness, especially along with stomach pain, nausea, or vomiting
  • Trouble breathing, rapid or deep breathing, or shortness of breath
  • Feeling unusually cold, or a slow, fast, or irregular heartbeat combined with dizziness or fainting
  • Vomiting or diarrhea so severe that you cannot keep fluids down, especially with confusion, fainting, or extreme weakness
  • Very little or no urine, especially with severe vomiting or diarrhea, dizziness, confusion, fainting, or marked weakness

See a doctor soon (same-day or next available appointment) if:

  • Stomach side effects that bother you a lot, last for more than a few weeks, come back after they have gone away, or start later in treatment
  • Stomach upset severe enough that you are eating much less than usual or skipping doses because of it
  • Weight loss that you did not expect or intend, or that is rapid or keeps going
  • Vomiting that keeps happening even though you are already taking metformin with food
  • New or worsening stomach symptoms that started right after a dose increase
  • Severe vomiting, diarrhea, or fever, or drinking much less than usual, because your prescriber may want to pause metformin until you recover
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Frequently Asked Questions

Is it normal to have diarrhea when starting metformin?

Yes, diarrhea is a common reaction when you first start metformin or raise the dose. In the placebo-controlled trial described in the FDA labeling for immediate-release Glucophage, more than half of the people taking it reported diarrhea. Symptoms often ease over the following weeks, but if diarrhea is severe or keeps returning, contact your prescriber, and if you cannot keep fluids down, get medical care right away.

How long does metformin stomach upset usually last?

It varies from person to person. The FDA patient information says these side effects generally go away after you take the medicine for a while, and symptoms are often most noticeable right after starting or raising the dose. Tell your prescriber if they bother you a lot, last for more than a few weeks, come back after going away, or start later in treatment.

Does taking metformin with food actually help?

Often, yes. The FDA patient information says taking metformin with meals can help reduce side effects, and immediate-release tablets are usually taken with meals two or three times a day. Extended-release tablets are usually taken once daily with the evening meal. Follow the directions on your prescription label and ask your pharmacist or prescriber about your specific product.

Should I ask about switching to extended-release metformin?

If immediate-release metformin causes ongoing diarrhea, nausea, or cramping even after you take it with meals, extended-release (ER or XR) metformin is a reasonable option to discuss with your prescriber. A 2022 systematic review found less bloating and diarrhea with the extended-release form, though it is not automatically right for everyone, so the switch should be made under medical guidance.

Can metformin cause stomach problems even after months of taking it?

Stomach symptoms are most common when starting or raising the dose, so new or worsening symptoms after months of stable use are worth reporting rather than assuming they are simply the metformin. A dose change, a new medication, reduced kidney function, or an unrelated digestive problem can produce similar symptoms, and your prescriber needs to know so the right cause gets identified.

Is it safe to stop taking metformin because of stomach upset?

Do not stop metformin on your own just because your stomach is upset. Your diabetes still needs to be managed while your gut adjusts, and stopping on your own can leave your blood sugar less well controlled. Call your prescriber instead. They can lower the dose, slow the titration schedule, switch you to extended-release metformin, or choose a different medication if metformin is not a good fit for you.

Why does metformin give some people a metallic taste?

A metallic or unpleasant taste is a recognized metformin side effect, and the exact reason is not fully understood. The FDA patient information says about 3 in 100 people notice it when they start metformin and that it lasts for a short time. If it persists or bothers you, mention it to your prescriber.

Can metformin cause weight loss along with the stomach upset?

Modest weight loss can occur in some people taking metformin, and for some people with type 2 diabetes a small amount of weight loss can be welcome. Rapid, continuing, or unintentional weight loss, especially alongside ongoing nausea or reduced appetite, should be mentioned to your prescriber.

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