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Diarrhea and other stomach problems are the most frequently reported side effects of metformin. About 53% of people on immediate-release metformin have experienced diarrhea in one clinical study. Studies also report that up to 46% of patients stop taking or do not consistently take metformin due to these digestive problems.

The timing of the diarrhea varies. Most often, it starts soon after you begin metformin or when your dose is increased. But interestingly, even long-term metformin use can cause diarrhea.

Below, we’ll explain why metformin can cause diarrhea, how long it can last, and most importantly, what you can do to relieve or prevent it.

Key Takeaways

  • Metformin frequently causes diarrhea, especially when you first start taking it. It’s more common with the immediate-release form than the extended-release form.
  • Metformin affects the digestive system in several ways. It alters gut bacteria, increases intestinal water content, and speeds up how fast food moves through your gut. These changes can lead to watery, frequent bowel movements.
  • Metformin diarrhea is often temporary. Many people find that the diarrhea improves after a couple of weeks as their body adapts. If your diarrhea lasts more than a few weeks or is very severe, let your doctor know.

Why does metformin cause diarrhea?

Several factors likely explain why metformin gives you diarrhea:

1. It changes your gut bacteria

Our digestive system contains trillions of bacteria, called the gut microbiome. Research suggests that part of metformin’s blood sugar-lowering effect may come from changing gut bacteria. One important change is an increase in a specific bacterium called Akkermansia muciniphila.

Studies in mice suggest that increasing Akkermansia muciniphila can:

    • reduce inflammation
    • improve gut barrier strength
    • improve glucose control
    • help the body handle high-fat diets better

However, changes in the gut microbiome may also cause stomach side effects.

A small study in people who struggled to tolerate metformin found that combining it with a gut microbiome modulator (GIMM), which helps support gut bacteria, reduced stomach side effects and lowered fasting blood sugar.

2. It causes bile acid issues

Our body makes bile acids in the liver to help digest fats. Normally, these bile acids are reabsorbed in the intestine and reused. But metformin reduces the reabsorption of bile acids, especially in the last part of the small intestine (the ileum). Because of this, more bile acids remain in the gut instead of being recycled.

When too many bile acids remain in the intestine, they can pull water into the gut, which can make your stool looser. That is one possible reason metformin can cause diarrhea.

3. It affects how your intestines absorb water

Diarrhea can occur when the intestine:

    • does not absorb nutrients properly, or
    • does not absorb enough salt and water

There is a protein in the intestine called sodium/hydrogen exchanger 3 (NHE3). Its job is to absorb sodium from the intestine into the body. When sodium is absorbed, water is absorbed with it, so the stool becomes more solid.

Researchers believe that metformin might reduce the activity of NHE3. In diabetic mice treated with metformin, experts observed more water remaining in the intestine and reduced activity of NHE3.

When NHE3 does not work properly, less sodium is absorbed, meaning more water stays in your intestines, making your stool softer.

4. It increases digestive hormones

Metformin can increase levels of certain hormones and signaling chemicals that affect digestion. One is GLP-1 (glucagon-like peptide-1), a hormone that slows stomach emptying.

Metformin may also influence serotonin and histamine. These chemicals help regulate digestion and the movement of food through the intestines. When serotonin or histamine levels rise, they can increase gut movement. Your intestines contract more and push contents through faster than usual, sometimes before it has a chance to form solid stool, contributing to diarrhea.

How long does metformin diarrhea last?

For most people, diarrhea usually improves within a couple of weeks as the body gets used to the medication. Many patients report that the worst symptoms are in the initial week or two. But by the end of the first month, things tend to settle down significantly.

One person shared on Reddit that their metformin-induced diarrhea “lasted about a month.”

Comment
byu/Leb_west from discussion
indiabetes_t2

If the diarrhea lasts longer than a few weeks, inform your doctor. Sometimes, persistent diarrhea could mean the dose is too high for you, or you might need additional help managing the side effect.

Can metformin cause diarrhea after taking it for years?

Yes. Doctors writing in journals reported cases of long-term metformin users developing diarrhea after years on the drug, and their symptoms went away only when metformin was stopped.

Case 1

A 63-year-old woman with long-standing type 2 diabetes lost about 10 kg (about 22 lbs) over one year and had intermittent watery diarrhea and indigestion. She was evaluated by several doctors and was initially thought to have diabetic diarrhea, possibly related to nerve damage.

However, her tests were normal. When doctors stopped her metformin, her symptoms improved within a week. Her diarrhea stopped, her stomach discomfort improved, and she eventually regained weight. Because her blood sugar later became difficult to control with other medications, she was switched to insulin therapy.

Case 2

A 74-year-old woman with diabetes had diarrhea triggered by meals for about 1.5 years, sometimes with fecal incontinence or losing control over bowel movements. After evaluation, she was diagnosed with irritable bowel syndrome (IBS) and given medication.

Despite treatment, the symptoms continued. When doctors stopped metformin, her diarrhea improved significantly, and she regained weight. A low-dose sulfonylurea was used instead to help control her blood sugar.

Case 3

A 62-year-old man with type 2 diabetes had been experiencing chronic diarrhea and unintentional weight loss of about 6-8 kg (13-18 lbs) for six years. His symptoms included frequent, painless stools, abdominal bloating, and cramping that usually occurred after meals, sometimes even waking him at night with an urge to use the bathroom.

His diabetes was well controlled with metformin (500 mg twice daily). He was also taking atorvastatin for cholesterol and telmisartan for hypertension.

The patient had already been evaluated by several doctors, but his symptoms did not improve. After stopping atorvastatin and telmisartan, his symptoms improved only slightly.

Doctors then suspected metformin-induced chronic diarrhea and stopped metformin for one month, replacing it with glimepiride. Within a few weeks, his diarrhea and bloating improved significantly, confirming that metformin was likely the cause of his symptoms.

Case 4

An 80-year-old man with type 2 diabetes developed chronic diarrhea for about 8 months, with 6-8 loose bowel movements per day and about 4.8 kg (10.6 lb) of weight loss. The diarrhea was severe enough that it limited his ability to leave the house.

He had been taking several diabetes medications, including metformin, gliclazide, and sitagliptin. His doctors performed many tests to look for possible causes of the diarrhea, but all results were normal. A colonoscopy also showed no significant abnormalities.

During a medication review, doctors noticed that his metformin dose had recently been increased from 500 mg twice daily to 1000 mg twice daily before the diarrhea started. Because no other cause was found, they asked him to stop metformin.

After discontinuing metformin, his diarrhea stopped almost immediately.

Older adults may have a higher risk of metformin-related diarrhea, both when starting the medication and later during treatment. This is partly because they often take multiple medications (polypharmacy), which can interact with metformin and increase its levels in the body.

Can metformin cause blood in stool?

Metformin rarely causes blood in stool directly. If it occurs, doctors usually investigate other gastrointestinal causes first before attributing it to the medication. Blood in the stool is more often caused by conditions such as:

    • Hemorrhoids
    • Anal fissures
    • Colon polyps
    • Inflammatory bowel disease
    • Gastrointestinal infections
    • Colon cancer

In a Reddit discussion, one user noticed bright red blood in their stool and asked if anyone else had experienced this while on metformin. Several users said blood in stool is not normal and should be checked by a physician.

The person who started the thread later updated that their bleeding was caused by hemorrhoids, which were treated with banding.

Blood in stool + metformin
by indiabetes_t2

While uncommon, blood in the stool should always be evaluated by a doctor, especially if it occurs with:

    • weight loss
    • persistent diarrhea
    • abdominal pain
    • black or tarry stools

How to prevent diarrhea when taking metformin

There are several proven strategies and home remedies that can help reduce diarrhea while on metformin.

1. Take metformin with meals

Always take your metformin dose with food, preferably during or right after a meal. Having food in your stomach slows down the absorption of the drug and cushions your digestive tract.

Also, be mindful of what you eat. Very greasy or sugary meals can sometimes make diarrhea worse. Some people find that lighter, low-carb meals cause less trouble. In fact, a few patients have noted that high-carbohydrate meals tend to trigger more “metformin poop,” whereas eating fewer carbs keeps things calmer.

2. Take metformin with probiotics or fiber supplements

A popular tip from patients is to take metformin with a bit of yogurt, especially Greek yogurt. The probiotic cultures in yogurt may help soothe the stomach. Several studies also support that probiotics may help reduce metformin-related digestive side effects, including diarrhea, nausea, and bloating.

You can get probiotics from fermented foods like yogurt, kefir, or sauerkraut, or take over-the-counter probiotic supplements.

Another idea is using fiber supplements to help bulk up stools. This may sound counterintuitive, since fiber can loosen stool. But, soluble fiber, like psyllium husk found in Metamucil or fiber gummies, can actually absorb excess water in the intestines and make stool more formed.

Several metformin users have reported success by taking a fiber supplement at the same time as their dose, saying “the only way I can take it is with added fiber” to avoid liquid stools.

Comment
byu/Leb_west from discussion
indiabetes_t2

Talk to your doctor first before trying probiotics, fiber supplements, or making changes to how you take metformin. Never stop or modify your medication without medical guidance.

3. Start low and increase slowly

When beginning metformin treatment, doctors often prescribe a low dose, such as 500 mg once a day, then gradually increase it to the full prescribed dose over several weeks. Do not take a high dose on your own.

If you are already on metformin and having trouble, ask your doctor if temporarily reducing the dose and re-titrating slowly is an option. It’s better to be on a slightly lower dose that you can tolerate than to quit the drug altogether.

4. Ask about extended-release metformin

Extended-release (ER) metformin, known by brand names like Glucophage XR or Glumetza, is formulated to dissolve slowly in your gut. This slower release leads to fewer gastrointestinal side effects for many patients.

A 2024 study compared two forms of metformin and found that metformin ER caused fewer digestive side effects than the immediate-release form.

In another study, 74.3% of patients who had gastrointestinal side effects from immediate-release metformin reported no side effects after switching to metformin ER. In addition, 93.8% were satisfied with the treatment change.

The dosing of ER might be slightly different, but the overall effectiveness in controlling blood sugar is the same. Insurance coverage and cost can vary, but low-cost generic metformin ER is widely available.

If you’re taking regular metformin and the diarrhea is unbearable, talk to your healthcare provider about switching to metformin ER.

5. Avoid trigger foods

Metformin’s effects on digestion can be aggravated by certain foods. While individual tolerances vary, high-insoluble fiber foods, very high-fat foods, and artificial sweeteners are common culprits that can worsen diarrhea.

Instead, opt for balanced meals with lean protein, moderate healthy fats, and controlled carbs. Also, consider smaller, more frequent meals rather than one huge meal with your medication. This can ease the burden on your digestive system.

Keep a food and symptom diary for a week or two if you’re not sure what your triggers are. Note what you ate, when you took metformin, and how your stomach reacted. You might see some patterns, and then you can adjust your eating habits accordingly.

6. Stay hydrated and replenish electrolytes

Diarrhea can dehydrate you, so it’s important to drink plenty of fluids when on metformin, especially if you’re having loose bowel movements every day. You could also lose electrolytes, like sodium and potassium. Sip on broths or electrolyte-replenishing drinks. Be cautious with sugary sports drinks, and choose low-sugar electrolyte solutions or coconut water.

Signs of dehydration include feeling very thirsty, dizzy, having a dry mouth, or having dark yellow urine. Make it a habit to drink water throughout the day.

7. Use over-the-counter relief sparingly

For short-term relief, you can consider over-the-counter (OTC) anti-diarrheal medicines like loperamide (Imodium). Loperamide works by slowing down gut movement and allowing more water to be absorbed, which can quickly firm up stools.

However, consult with your doctor before relying on anti-diarrheal meds regularly. These medications treat the symptom (diarrhea) but don’t fix the underlying cause, and using them too often could mask a problem or cause constipation instead. 

Another OTC option some people use is Pepto-Bismol or bismuth subsalicylate, which can ease diarrhea and cramping. Again, get medical advice if you find yourself needing these remedies frequently. Never take an anti-diarrheal if you suspect you have an infection.

8. Be open with your doctor

It’s important your provider knows if the side effects are impacting your life. They can adjust your treatment plan in a number of ways. They might reduce your metformin dose, switch you to the extended-release version (if not tried yet), or even add a second medication to help control blood sugar so you can stay on a lower metformin dose.

In some cases, if metformin just isn’t tolerable no matter what, your doctor may opt to use a different class of diabetes medication for you. There are plenty of alternatives, such as:

Your doctor will choose the best option based on factors such as your blood sugar levels, other health conditions, weight goals, and risk of side effects.

They might also check if something else is contributing to the diarrhea. For example, uncontrolled blood sugar itself can cause more urination and diarrhea in some diabetics, and high sugar levels can lead to yeast overgrowth or infections that upset the gut. Your doctor can evaluate if your diarrhea is truly from metformin or if another issue needs addressing.

Bottom line

Metformin can cause diarrhea, and it’s actually one of its most common side effects. It can happen because of how metformin interacts with your gut, altering gut bacteria, causing bile acid issues, and increasing digestive hormones, all of which can result in loose, watery stools.

Fortunately, this side effect is often temporary and improves after a few weeks as the body adjusts. If symptoms persist, strategies such as taking metformin with food, starting at a low dose, or switching to an extended-release version may help manage the problem. Always discuss symptoms with your healthcare provider, who can help guide you through solutions.

Frequently Asked Questions

Yes, in many cases, metformin-related diarrhea improves after the body adjusts to the medication. This usually happens within the first few weeks of treatment. Taking the drug with food or switching to an extended-release version may also help reduce symptoms.

The most frequently reported side effects are diarrhea and nausea. These gastrointestinal symptoms occur because metformin affects digestion and the gut microbiome. They are usually mild and tend to improve as treatment continues.

Yes, several diabetes drugs can cause diarrhea, particularly metformin and some GLP-1 receptor agonists. These medications influence gut activity, digestion, and hormone signaling in the digestive tract. The severity varies depending on the drug and the individual.

High-fat, very sugary, or highly processed foods can worsen digestive side effects while taking metformin. Large meals or foods that irritate the stomach may also trigger loose stools. Eating balanced meals with soluble fiber and moderate carbohydrates often helps reduce symptoms.

Many people remain on metformin for years because it is commonly used as a first-line treatment for type 2 diabetes. As long as blood sugar is controlled and side effects are manageable, doctors may continue it long term. Some patients take it for life unless another therapy replaces it.

Consuming large amounts of sugar can raise blood glucose levels even if you are taking metformin. The medication helps improve insulin sensitivity, but it cannot fully counteract excessive sugar intake. Regular high-sugar meals may reduce the overall effectiveness of treatment.

Metformin is usually taken with meals to reduce stomach upset. If prescribed once daily, many doctors recommend taking it with the evening meal. For twice-daily dosing, it is commonly taken with breakfast and dinner.

Metformin is not known to cause high blood pressure. In some studies, it has even been linked to modest improvements in cardiovascular risk factors. Blood pressure changes can occur for many other reasons.

Jardiance (empagliflozin) is typically taken once daily in the morning, with or without food. Metformin is usually taken with meals to minimize gastrointestinal side effects. When using Jardiance and metformin together, it’s best to take Jardiance in the morning and metformin with breakfast and/or dinner, depending on your dosing schedule.

Both metformin and Ozempic commonly cause gastrointestinal side effects. Because Ozempic is given by injection and reaches higher active levels in the body, its effects, including digestive symptoms, may feel more pronounced for some people.

Related Articles

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