Yes, diarrhoea can sometimes appear after you start taking a probiotic, but the timing alone does not prove that the probiotic caused it. Probiotics affect the microorganisms and activity in the digestive system, and individual products contain different strains, doses and additional ingredients. At the same time, probiotics are also studied for preventing or reducing some types of diarrhoea, including antibiotic-associated diarrhoea, so the relationship is not straightforward.
If you develop loose or watery stools after starting a probiotic, consider the probiotic alongside any other supplements, medicines, dietary changes or recent illnesses. The related parent guide on probiotic side effects and the wider Supplement Side Effects hub can help you understand other possible digestive reactions.
Most short episodes of diarrhoea are not emergencies, but persistent diarrhoea, dehydration, blood in the stool, severe abdominal pain or other concerning symptoms should be assessed by a healthcare professional.
How probiotics Causing Diarrhoea can be linked to medicines or supplements
Possible medicine categories
When diarrhoea starts after a probiotic, it is important not to assume that the probiotic is the only possible cause.
A doctor or pharmacist may want to know about everything you are taking because diarrhoea is a recognised side effect of many medicines and supplements. Examples include:
- Antibiotics. Diarrhoea is a common antibiotic side effect. Antibiotics can alter the normal balance of organisms in the bowel, and in some cases antibiotic-associated diarrhoea can become more serious.
- Metformin. This commonly used diabetes medicine can cause diarrhoea and other digestive symptoms in some people.
- Magnesium-containing supplements or medicines. Higher amounts of supplemental magnesium can cause diarrhoea, nausea and abdominal cramping.
- Laxatives. These medicines are intended to loosen stools, and excessive use can cause problematic diarrhoea.
- Other medicines and supplements. A wide range of products can contribute to diarrhoea, including some medicines used for stomach conditions, certain anti-inflammatory medicines and other supplements.
If your probiotic product contains other ingredients, those also matter. For example, some products contain prebiotic ingredients as well as probiotics. Higher intakes of certain prebiotics can cause gastrointestinal symptoms including bloating, cramping and diarrhoea.
Timing clues
The timing of your symptoms can help a healthcare professional investigate what may be happening.
Make a note of whether the diarrhoea:
- started soon after you began the probiotic;
- appeared after increasing the amount you take;
- began after switching to a different probiotic formulation;
- started at the same time as an antibiotic or another medicine;
- followed a change in diet or another supplement;
- is accompanied by vomiting, fever, abdominal pain or blood in the stool; or
- continues even when no other digestive symptoms are present.
Timing can suggest a possible connection, but it cannot confirm the cause. Viral gastroenteritis, foodborne illness, another medicine or an underlying digestive condition may also cause diarrhoea.
Medicines and product categories that may be associated with this symptom
Probiotic supplements are not all interchangeable. Different products can contain different species and strains of microorganisms, different quantities, and other added ingredients. Research on probiotic safety also varies considerably between products, and there is limited information about the frequency and severity of side effects across every probiotic formulation.
This means that a reaction to one product does not necessarily predict how you would respond to another product. It also means you should avoid assuming that a particular strain is responsible without professional assessment.
Other products being taken at the same time can be particularly important. For example, someone who starts a probiotic during a course of antibiotics may develop diarrhoea because of the antibiotic, an infection, the probiotic, another ingredient, or a combination of factors. Probiotics themselves have also been studied as a way of reducing antibiotic-associated diarrhoea, which is another reason causality should not be assumed from timing alone.
If you are taking several medicines or supplements, review the medication side effects guide and discuss your complete medicine and supplement list with a pharmacist or doctor.
Red flags: when this symptom may need urgent medical attention
Red flags
Diarrhoea can cause dehydration, particularly when stools are frequent or you are also vomiting. Children, older adults and people with weakened immune systems can be more vulnerable to complications from dehydration.
Seek urgent medical advice if diarrhoea is accompanied by:
- blood or pus in the stool;
- black, tar-like stools;
- severe or persistent abdominal or rectal pain;
- repeated vomiting that makes it difficult to keep fluids down;
- significant dizziness or fainting;
- very little urine or unusually dark urine;
- marked weakness, drowsiness or confusion;
- a high fever; or
- symptoms that are rapidly worsening.
Adults should also contact a healthcare professional if diarrhoea continues for more than about two days without improvement. Children need earlier assessment because dehydration can develop more quickly.
Get immediate emergency medical help for severe difficulty breathing, collapse, seizures, confusion or other signs of a medical emergency.
People who are seriously ill or have significantly weakened immune systems should be particularly cautious about probiotic use. Although serious complications are uncommon, infections associated with probiotic microorganisms have occasionally been reported in high-risk patients.
What to track before speaking to a doctor or pharmacist
What to track
If you are concerned about probiotics causing diarrhoea, a short symptom record can make a conversation with your doctor or pharmacist much more useful.
Write down:
- the exact probiotic product and brand;
- the strains listed on the label, if available;
- how much you take and how often;
- when you started it;
- whether you recently changed the product or amount;
- when the diarrhoea began;
- approximately how many loose stools you are having each day;
- whether there is blood, mucus or unusual stool colour;
- whether you have cramps, fever, nausea or vomiting;
- any signs of dehydration;
- all prescription medicines you take;
- over-the-counter medicines, vitamins and other supplements;
- any recent antibiotic treatment;
- recent travel, dietary changes or possible foodborne illness; and
- whether the symptoms are improving, worsening or staying the same.
Taking the probiotic container or a clear photo of its ingredient label to your pharmacist or appointment can also help identify ingredients that may be relevant.
Keeping track of the sequence of events is especially useful. For example, “I started an antibiotic on Monday, added a probiotic on Wednesday and developed diarrhoea on Thursday” gives a healthcare professional more useful information than simply saying that the probiotic caused the symptom.
What not to do without medical advice
What not to change without advice
Do not stop or alter a prescribed medicine simply because diarrhoea started after you began taking a probiotic. The prescribed medicine may itself be contributing to the symptom, but stopping some treatments without advice can cause other problems.
Speak to the prescribing doctor or a pharmacist about what you are experiencing and ask whether any medicine, supplement or combination needs to be reviewed.
You should also avoid repeatedly adding new supplements in an attempt to “balance” your digestive system while you are trying to work out what is causing the problem. Changing several products at once can make the pattern harder to assess.
Maintaining fluids is important when you have diarrhoea. Signs such as dark urine, reduced urination, dizziness, dry mouth and unusual tiredness may indicate dehydration. A pharmacist can advise whether an oral rehydration solution is appropriate for you.
Be cautious about self-treating significant diarrhoea with anti-diarrhoeal medicines without advice, particularly if you have fever, bloody stools or severe symptoms. Some infections and medical conditions require a different approach.
Related side-effect guides and reporting options in South Africa
If you are trying to work out whether diarrhoea may be related to a supplement, medicine or another treatment, you can explore the side effect symptoms A-Z guide for related symptoms and safety information.
If you suspect that a medicine or health product has caused an adverse effect, patients and healthcare professionals in South Africa can report suspected adverse events to the South African Health Products Regulatory Authority (SAHPRA). SAHPRA accepts reports from consumers as well as healthcare professionals through its medicine-safety reporting systems.
See our guide on how to report side effects in South Africa for the local reporting route.
Is diarrhoea a common side effect of probiotics?
Digestive changes can occur when taking supplements that affect the gut, but available research does not allow a reliable estimate of how often probiotics themselves cause diarrhoea across all strains and products. Probiotics are generally considered to have a history of apparently safe use in healthy people, while detailed safety data remain limited.
Because probiotics are also used or studied to help prevent some types of diarrhoea, new diarrhoea should not automatically be attributed to the probiotic.
How long can diarrhoea last?
Many uncomplicated episodes of acute diarrhoea improve within a few days. If you are an adult and symptoms continue for more than about two days without improvement, or they are becoming worse, contact a healthcare professional. Seek help sooner if you have dehydration, blood or black stools, severe pain, high fever or another warning sign.
Should I stop the medicine or probiotic?
Do not stop a prescribed medicine without advice from the clinician who prescribed it or another qualified healthcare professional. If you suspect your probiotic or another non-prescription product is contributing to diarrhoea, a pharmacist or doctor can help assess the product, your symptoms and anything else you are taking before you make changes.
When should I seek help?
Speak to a doctor or pharmacist if diarrhoea is persistent, worsening, unusual for you or difficult to manage, or if you are concerned about a medicine or supplement.
Seek urgent care for blood in the stool, black stools, severe abdominal pain, significant dehydration, repeated vomiting, fainting, confusion or other severe symptoms. Babies, young children, older adults, pregnant people and people with weakened immune systems or serious medical conditions should generally receive medical advice earlier.
This information is for general education and does not diagnose the cause of diarrhoea or replace individual medical advice. If you are concerned about symptoms after taking a probiotic, medicine or supplement, keep a record of what you have taken and when the symptoms started, and speak to a doctor or pharmacist.