Yes. Diarrhoea is a recognised and common gastrointestinal side effect of metformin. South African professional information for metformin products lists diarrhoea alongside nausea, vomiting, abdominal pain and loss of appetite, and international medicine-safety sources also identify diarrhoea as a common metformin side effect.
Metformin may therefore be a possible explanation if diarrhoea begins while you are taking it, particularly around the time treatment starts or the dose or formulation changes. However, diarrhoea has many other causes, including infections, food-related illness, other medicines and supplements. Having diarrhoea while taking metformin does not automatically prove that metformin is the cause.
For a broader look at digestive symptoms linked to medicines, see our guide to medicines that can cause diarrhoea or browse the Side Effect Symptoms hub.
How metformin diarrhoea can be linked to medicines or supplements
Metformin is widely used to help manage blood glucose, particularly in people with type 2 diabetes. Gastrointestinal side effects are among the best-known problems associated with the medicine. South African professional information specifically lists diarrhoea as a frequent gastrointestinal adverse effect of metformin.
Timing can provide useful clues
If diarrhoea starts soon after beginning metformin, after a dose increase or following another change in treatment, that timing is worth mentioning to your doctor or pharmacist. Some metformin product information notes that increasing the dose gradually may improve gastrointestinal tolerability. Any dose changes, however, should be made under professional guidance rather than on your own.
Diarrhoea that starts much later should not automatically be blamed on metformin. A stomach infection, food poisoning, another medicine, a supplement or an unrelated digestive condition could also be responsible.
Useful questions include:
- Did the diarrhoea start before or after a change in metformin treatment?
- Is it happening every day or only occasionally?
- Does it tend to occur after a particular dose or meal?
- Have any other medicines or supplements recently been started?
- Are there symptoms such as vomiting, fever, severe abdominal pain or blood in the stool?
These details can help a healthcare professional assess whether metformin is a likely contributor or whether another cause needs to be considered.
Medicines and product categories that may be associated with this symptom
Metformin is not the only medicine that can cause diarrhoea. MedlinePlus notes that drug-induced diarrhoea can occur with a wide range of medicines, including antibiotics, magnesium-containing antacids, laxatives, some anti-inflammatory medicines, certain medicines that suppress the immune system and some cancer treatments. Metformin is also specifically listed as a medicine associated with diarrhoea.
Other medicines and supplements matter
When discussing diarrhoea with a pharmacist or doctor, provide a complete list of what you use, including:
- prescription medicines;
- over-the-counter medicines;
- vitamins and minerals;
- herbal or complementary products;
- laxatives;
- antacids;
- recently completed courses of antibiotics.
Some herbal products can also have a laxative effect. For example, products containing senna may cause diarrhoea. Magnesium-containing products may also contribute in some people.
Do not assume that because metformin is known to cause diarrhoea it must be responsible for every episode. Viral gastroenteritis, contaminated food or water and other digestive problems are also common causes.
Red flags: when this symptom may need urgent medical attention
Mild diarrhoea may settle, but more severe or persistent diarrhoea can lead to dehydration. Dehydration deserves particular attention in someone taking metformin because severe diarrhoea, vomiting, fever or substantially reduced fluid intake can affect the body’s fluid balance and may increase medicine-related risks. MedlinePlus specifically advises people taking metformin to tell their doctor if they develop severe diarrhoea, vomiting, fever or markedly reduced fluid intake.
Get medical advice promptly if you have
Contact a doctor or other appropriate healthcare professional if:
- diarrhoea is severe or continues rather than improving;
- you are struggling to drink enough fluid;
- you are urinating much less than usual;
- your urine becomes unusually dark;
- you have repeated vomiting;
- you have significant or persistent abdominal pain;
- you develop a high fever;
- there is blood or pus in your stool;
- stools are black or tar-like;
- you feel unusually weak, dizzy or faint.
Diarrhoea can cause dehydration, which can become serious, particularly in older adults and people with other medical conditions.
Seek emergency medical help for severe symptoms
Metformin is associated with a rare but serious condition called lactic acidosis. It is uncommon, but it requires urgent medical treatment. Symptoms described in medicine information can include severe weakness or tiredness, unusual sleepiness, difficulty breathing, feeling unusually cold and significant stomach or intestinal symptoms.
Seek immediate medical attention if diarrhoea occurs together with severe breathing difficulty, collapse or fainting, marked confusion, severe weakness, chest pain, seizures, or swelling of the face, lips, tongue or throat.
Do not delay emergency care while trying to decide whether metformin is responsible.
What to track before speaking to a doctor or pharmacist
A simple symptom record can make a medical or pharmacy consultation more useful.
Keep track of the diarrhoea
Record:
- when it started;
- how many loose or watery stools you have each day;
- whether symptoms are improving or worsening;
- whether there is blood, mucus or an unusual stool colour;
- whether you also have nausea, vomiting, cramps or fever;
- how much fluid you are managing to drink;
- whether you are urinating normally.
Record your medicine details
It can also help to note:
- the name and strength of your metformin;
- whether it is a standard or extended-release formulation;
- your usual dosing schedule;
- when your current dose was started;
- any recent dose changes;
- other diabetes medicines you use;
- newly started prescription or non-prescription products;
- vitamins, minerals and herbal supplements.
If your healthcare team has asked you to monitor your blood glucose, keep those readings available as well.
You do not need to decide for yourself whether the diarrhoea is definitely caused by metformin. The aim is to give your healthcare professional enough information to make a safer assessment.
What not to do without medical advice
If you suspect metformin is causing diarrhoea, do not stop, reduce, increase or otherwise change your prescribed dose on your own. Metformin is often being used to control an ongoing condition, and changes to treatment should be discussed with the doctor or pharmacist responsible for your care.
Do not automatically treat diarrhoea with another medicine
The NHS advises people experiencing metformin-related diarrhoea not to take medicines to treat the diarrhoea without first speaking to a doctor or pharmacist.
That is important because the right response depends on the cause, severity, other medicines you take and your general health.
Pay attention to hydration
Fluid loss is one of the main concerns with ongoing diarrhoea. Drinking fluids can help reduce the risk of dehydration, but anyone who cannot keep fluids down, is becoming increasingly weak or is urinating much less than usual should obtain medical advice promptly.
If you use hormonal contraceptive pills, severe diarrhoea may also affect contraceptive protection in some circumstances. Check the instructions supplied with your contraceptive and speak to a pharmacist or healthcare professional if you are unsure.
Related side-effect guides and reporting options in South Africa
If you are trying to work out whether another treatment might be contributing to your symptoms, explore our broader guide to medication side effects and the side effect symptoms A–Z hub.
Reporting a suspected metformin side effect
South Africans can report suspected adverse reactions to the South African Health Products Regulatory Authority (SAHPRA). SAHPRA states that reports from both consumers and healthcare professionals contribute to medicine-safety monitoring, and suspected adverse events can be submitted through its reporting systems.
See our guide on how to report side effects in South Africa.
Reporting a suspected side effect does not prove that the medicine caused it. It helps regulators monitor possible safety signals across larger numbers of medicine users.
Is diarrhoea a common side effect of metformin?
Yes. Diarrhoea is a well-recognised gastrointestinal side effect of metformin. South African professional information lists diarrhoea among frequent gastrointestinal adverse effects, and other recognised medicine-information sources also classify it as a common metformin side effect.
How long can metformin diarrhoea last?
There is no single duration that applies to everyone. Some gastrointestinal symptoms may improve as treatment continues, while persistent or severe diarrhoea needs medical assessment. If diarrhoea continues, repeatedly returns or is affecting your ability to eat, drink or carry out normal activities, speak to your doctor or pharmacist.
Should I stop metformin if I have diarrhoea?
Do not stop or change a prescribed medicine without professional guidance unless you are following specific instructions already provided by your healthcare professional or the medicine’s official patient information.
Speak to your doctor or pharmacist if diarrhoea is troublesome, persistent or worsening. Severe diarrhoea, vomiting or dehydration deserves prompt medical advice because these problems may affect the safe use of metformin.
When should I seek help?
Seek medical advice if the diarrhoea is severe, persists, is accompanied by repeated vomiting, fever, significant abdominal pain, blood or black stools, or signs of dehydration.
Seek emergency help for symptoms such as severe breathing difficulty, fainting, marked confusion, seizures, severe weakness, chest pain or swelling of the mouth, tongue or throat.
Medical disclaimer: This information is for general educational purposes and does not diagnose the cause of diarrhoea or replace advice from a doctor, pharmacist or other qualified healthcare professional. If symptoms are severe, worsening, unusual or you are concerned about taking metformin safely, obtain professional medical advice.