Metformin and Diarrhoea – How to Fix Tummy Side Effects
Medically reviewed by Dr Sultan Linjawi, Endocrinologist & Diabetes Specialist — December 2025
It is well known that metformin can cause diarrhoea and this is one of the most common side effect that people experience (The Prevalence of Chronic Diarrhea Among Diabetic Patients). It should ‘settle’ after starting on the medication. But what if it doesn’t? What does ‘settle’ actually mean and how do you know if you should continue taking it? Also, why should you actually take something which causes such symptoms?
What is Metformin?
Metformin is a type of medication used to treat high blood glucose level in people with type 2 diabetes. It comes in the form of a pill with some brand names including Diabex, Glucophage, and Diaformin. It is often the first drug to be prescribed to people with diabetes. Metformin can also be used in women with gestational diabetes and polycystic ovary syndrome (PCOS) with or without diabetes.
What Other Names is Metformin Known By?
Metformin is the official or generic name of the drug. It is sold by many manufactures under many different names including, but not limited to,
- Diabex
- Diaformin
- Diabex
- Fortamet
- Glucophage
- Glumetza
- Riomet
How does Metformin Work?
Metformin works by lowering the amount of glucose that is made by the liver, which can result in the body’s cells being more sensitive to the action of insulin (Metformin—mode of action and clinical implications for diabetes and cancer). The United Kingdom Prospective Diabetes Study (UKPDS) found that as a result of using metformin, patients had better glycaemic control as metformin was able to slightly decrease blood glucose levels in the blood.
Metformin may also reduce fats in the blood, including lowering cholesterol and triglycerides (Effects of Metformin on Metabolite Profiles and LDL Cholesterol in Patients With Type 2 Diabetes).
How do I Take Metformin?
When someone with diabetes first starts taking metformin, typically it’s started as a once-daily dose with dinner (or your last meal). After one to two weeks, a second daily dose of metformin is added (taken at breakfast time). When taking metformin, it should be taken at about the same time every day and with a glass of water.
Recently metformin has been released as extended release or slow release versions. These are can often be recognised as they have XR or ER after their name. These extended release versions add the benefit of only needing to be taken once a day. They are reported to produce less side effects. These pills are in a special capsule that has tiny perforations in the wall of the capsule. This allows the drug to trickle as the capsule moves down the gut. Often the capsule is seen in a person's faeces and people may be concerned that the drug is not be absorbed. It is being absorbed but the outer casing is not.
How does Metformin cause Diarrhoea?
The most common side effects of metformin are gastrointestinal side effects. This is because one of the ways that metformin works is directly on the gut leading to symptoms of nausea and diarrhoea, especially in the first few of weeks taking it. That’s why you should start on a low dose to get used to it and also why most preparations are now slow-release. Only about 5% of those taking it actually have to stop long term due to this side effect as in the majority of people it either resolves or is ‘manageable’.
What are the Other Side Effects of Metformin
Including diarrhoea and nausea, other gastrointestinal side effects include the following:
- a metallic taste in your mouth
- bloating and flatulence
- some weight loss
- vomiting
- abdominal discomfort (Metformin and digestive disorders).
These symptoms are often mild, may come and go, and will go away after you stop taking metformin.
What is Diarrhoea and How Long Does it Last?
Looking through search engines such as Google and other websites you will find that there is little information relating to how long it takes for diarrhoea to settle. In fact, the truth is that most people have a different view of what diarrhoea actually is and if they can tolerate it. Loose stool may mean 3 or 4 times a day or less (or more) and not be an issue. Liquid, explosive episodes with little warning on the other hand can definitely impact daily life and lead to other health problems and dehydration. Some people prone to constipation may actually not mind being a little loose!
The important thing is to recognize when it is a problem, and, if it could be the sign of something else such as infection or even cancer. One way to find out if it is the metformin is to stop taking it for a few days. If you still have diarrhoea after 3 or 4 days then you have your answer. If you have not ever had a problem and suddenly are aware that your bowel habits have changed then it could indicate a serious problem and you definitely need to get that checked out with your doctor.
Is There a Way to Improve Diarrohea Caused by Metformin?
For most people yes there is. Diarrhoea is usually caused by the dose you are taking and how long you are taking it for. In general it settles for most people and the benefits outweigh any issue they have.
My normal recommendation as a diabetes specialist is to start at a low dose (typically 500mg daily) and increase the dose after 2-3 weeks to 1000mg. Sometime if symtpoms persist waiting another week or 2 is helpful. Over time i aim to get my patient on 2000mg(or 2g) daily in divided doses.
Some people may only be able to tolerate a small dose so if you increase the dose but diarrhoea does not settle after a few weeks then maybe you need a smaller dose and you should let your doctor know.
Are there any other side effects of metformin?
Yes.
Vitamin B12 Deficiency
Another common side effect of metformin use is vitamin B12 deficiency. Vitamin B12 has many important functions in our bodies. It’s needed to help make new blood cells and help our brain and nervous system to function properly.
About 30% of people taking metformin experience a reduced ability to absorb B12 from the intestine and this results in about 5-10% having a B12 deficiency (Vitamin B12 deficiency and diabetes). A low B12 level can result in fatigue and other symptoms seen in anaemia but also causes peripheral neuropathy- pins and needles or numbness in the feet or hands. B12 levels can be checked via routine blood tests. B12 supplements are available by a small monthly injection over 3 months if levels are depleted as it is difficult to get enough in our diet to increase levels.
Metformin can interfere with the absorption process of vitamin B12, which can cause people with diabetes to become deficient in B12. If you haven't been tested for B12 deficiency, request a blood test next time you see your doctor. Treatment for vitamin B12 deficiency is super simple and includes either a intramuscular injection or oral supplementation. Your doctor will choose the best treatment option for you.
For more information, read the article on Vitamin B12 Deficiency and Diabetes.
Nausea
Nasuea is another common gastrointestinal side effect of metformin. This affects about 7% of people but is reported to be higher if not taking a slow release formula (definitely worth checking out what you have been prescribed) (Metformin in Noninsulin‐Dependent Diabetes Mellitus). If it is continuing then it is not normal and will need to be discussed with your doctor. It is important to have this medication at mealtimes to minimize these effects.
Metallic Taste
A few people complain of a metallic taste (up to 3%) (Metformin in Noninsulin‐Dependent Diabetes Mellitus) and this should resolve once your body gets used to the medication. If it continues talk to your doctor.
Odour
Some people have noticed an odour from the metformin tablets that is fishy or like a “locker room sweat”. Again, this is an unusual experience but If concerned speak to your pharmacist as a different brand can be given.
But why should I take Metformin?
So, I guess by now you are wondering why on earth you should take Metformin at all…
It is actually a really good medication that works not only to help regulate blood glucose levels but also decreases the amount of insulin you need. Slowing the actual progression of diabetes, it has also been linked to other health benefits such as reducing cardiovascular events such as heart attacks and strokes; lowering cholesterol levels and even protecting against cancers. For more information please refer to Benefits of Metformin.
Studies indicate that it should be the drug of first choice on diagnosis of type 2 diabetes for very good reasons (American Diabetes Association: Approaches to Glycemic Treatment). When someone is given the diagnosis of diabetes, metformin is the first oral drug to be prescribed alongside lifestyle interventions, including changes in diet and physical activity levels. It is also used in pre-diabetes, gestational diabetes and polycystic ovarian syndrome and is considered a safe medication to take with few side effects other than those affecting the gut.
It is important therefore to first weigh up the pros and cons of taking metformin and talk to your doctor before stopping it to ensure you are making the best choice for your diabetes and general health. It may be as simple as a reduction in dose or changing brands to make a real difference.
Talk to your doctor
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Oral Medications for Diabetes
Metformin is the most commonly prescribed oral medication for type 2 diabetes. If you would like a clear overview of what metformin is, how it works, and why it is usually recommended first, start here: What is metformin?
Do you want to know more about Metformin?
Check out our articles for more content on metformin.
Frequently Asked Questions
What is metformin?
Metformin is an oral medicine commonly used as first-line treatment for type 2 diabetes. It lowers blood glucose by improving insulin sensitivity and reducing glucose production in the liver. Brands include Diabex, Glucophage, and Diaformin. It may also be used in gestational diabetes and polycystic ovary syndrome under clinician guidance.
How does metformin work?
Metformin mainly reduces the amount of glucose the liver releases into the bloodstream. It also improves how the body responds to insulin and may modestly improve cholesterol and triglyceride levels in some people.
How should I take metformin?
Metformin is usually started once daily with the evening meal and increased over one to two weeks to reduce stomach upset. Standard tablets are often taken twice daily. Extended-release formulations can be taken once daily and may reduce gastrointestinal side effects.
Why does metformin cause diarrhoea?
Metformin commonly causes gastrointestinal symptoms such as diarrhoea, bloating, or nausea—especially when treatment starts or the dose increases. Taking it with food, starting with a lower dose, or switching to an extended-release version can significantly improve tolerance.
How long does diarrhoea from metformin last?
Mild diarrhoea often settles within days to weeks as the body adapts. Persistent, severe, or worsening symptoms should be reviewed by a clinician, particularly if there is dehydration, fever, bleeding, or unintentional weight loss.
What other side effects can metformin cause?
Long-term use may lower vitamin B12 levels, which can lead to anaemia or symptoms such as numbness and tingling. Other possible effects include abdominal discomfort, metallic taste, or reduced appetite. A simple blood test can check B12 levels.
Why should a person take metformin?
Metformin helps improve blood glucose control, reduces insulin resistance, and can delay the need for insulin therapy. It is safe, effective, and supported by strong clinical evidence for most people with type 2 diabetes.
Who is metformin recommended for?
Most adults with newly diagnosed type 2 diabetes are recommended metformin as their first medication, unless contraindicated. It may also be used in prediabetes, gestational diabetes, and PCOS with specialist guidance.
Can I drink alcohol while taking metformin?
Alcohol should be consumed in moderation. Excessive drinking increases the risk of lactic acidosis and can cause low blood glucose, especially when combined with other diabetes medicines.
Can metformin be taken during pregnancy?
Metformin is used in some pregnancies, such as gestational diabetes, depending on local guidelines. Diabetes in pregnancy requires personalised specialist care and regular monitoring.
When should metformin not be taken?
Metformin may be unsuitable for people with severe kidney impairment, significant liver disease, dehydration, acute illness, or conditions that reduce oxygen supply to tissues. It may need to be paused before certain surgeries or contrast imaging tests. Always follow your clinician’s advice.
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