Probiotics.

A category, not a single intervention

Products contain different microorganisms or mixtures. They may differ in strain, amount and storage needs. A species name or a high count alone does not tell you whether a product helps a particular condition.

A label can identify the genus, species and strain. For example, “GG” identifies the strain in Lacticaseibacillus rhamnosus GG. A study of that strain does not automatically establish a benefit for another strain or mixture. Labels may also list colony-forming units (CFUs), an estimate of live microorganisms per serving. More CFUs do not automatically mean more benefit.

Check whether the count applies at manufacture or through the use-by date, and follow the product’s storage instructions. The count is a manufacturer claim; it does not independently prove the live count after unknown storage conditions or a clinical benefit. See probiotic storage and how to read a supplement label.114

Read the evidence by outcome

Some probiotics have been studied for problems such as diarrhoea associated with antibiotics. Results and product details matter: a finding in one group or formulation does not automatically apply to another. Ask what was tested, what it was compared with and for how long.1

Can probiotics prevent diarrhoea during antibiotics?

A 2021 review combined 42 randomised studies involving 11,305 adults taking antibiotics. Across varied probiotic preparations and comparisons, it found a lower average risk of antibiotic-associated diarrhoea. Its authors rated the overall certainty moderate.

That average does not mean every strain works or everyone benefits. The number of people who might benefit also depends on how likely diarrhoea was without the product. A health professional who knows your health and medicines can discuss a particular product.2

Why did one large trial find no benefit?

In the 2013 PLACIDE trial, nearly 3,000 hospital patients aged 65 or older who were taking antibiotics were randomly assigned a particular mixture of strains or a placebo. Antibiotic-associated diarrhoea occurred in 159 of 1,470 people analysed in the probiotic group (10.8%) and 153 of 1,471 in the placebo group (10.4%). No meaningful difference was established.

This does not disprove every probiotic. It shows that results from other mixtures or groups cannot simply be assumed to apply here. The UK National Institute for Health Research funded the trial.3

Do probiotics restore the microbiome after antibiotics?

Antibiotics can change the community of microorganisms living in the gut. A 2024 review of seven studies found mixed results for probiotics: some measurements suggested less disruption, while others showed no clear effect or delayed recovery. Products, antibiotics and laboratory methods varied. There is no single agreed definition of a restored or healthy microbiome. A change in microbes or their diversity does not automatically mean symptoms or future health improve.

A 2026 trial tested a specific combination of live bacteria and a plant-derived ingredient, called a synbiotic, against placebo in healthy adults who also received antibiotics. Of 32 people assigned, 11 did not complete follow-up; 21 were included in the analysis. Researchers reported changes in selected microbes and laboratory markers. The small study does not establish that the product restores everyone’s microbiome or prevents illness. The product company, Seed Health, funded it, and authors included company employees. Its size and missing participants add uncertainty.

Whether a particular product reduces antibiotic-associated diarrhoea is a separate question, discussed above. Do not stop or change prescribed antibiotics because of a microbiome claim. A pharmacist or clinician can discuss your circumstances, especially if your immune system is weakened.1213

Do probiotics prevent C. diff infection?

Not all diarrhoea during or after antibiotics is caused by C. diff, a bacterium that can cause infection and inflammation of the bowel. Preventing antibiotic-associated diarrhoea in general is a different research question from preventing C. diff specifically.

In the PLACIDE trial described above, C. diff diarrhoea occurred in 12 of 1,470 participants given the studied probiotic mixture and 17 of 1,471 given placebo. The trial did not establish a protective effect; relatively few people developed this outcome, leaving uncertainty about the size and direction of any difference. It tested one preparation in older hospital patients, not every product.

US professional guidelines have reached different conclusions. In 2020, the American Gastroenterological Association suggested certain specific strains or combinations for prevention in adults and children taking antibiotics. In 2021, the American College of Gastroenterology recommended against probiotics for preventing a first C. diff infection during antibiotics and against their use to prevent recurrence. In 2026, the AGA separately advised against probiotics for prevention in people with inflammatory bowel disease. These recommendations concern distinct patient groups and assessments of the evidence; none establishes that any shop-bought probiotic will prevent or treat an individual infection. People with serious illness or weakened immunity may face additional risks from live organisms.

The US CDC advises contacting a healthcare professional about diarrhoea during or after antibiotics. Do not rely on a probiotic to diagnose or treat possible C. diff, or stop prescribed antibiotics without discussing them with the prescriber.3891011

Can probiotics help irritable bowel syndrome?

Irritable bowel syndrome (IBS) involves abdominal pain and changes in bowel habits. Some studies suggest probiotics may help symptoms, but the evidence does not identify a reliably effective product for everyone.

A 2023 review of six placebo-controlled trials found no statistically significant improvement in its combined IBS severity score. Pooled results suggested less pain and bloating, but differences between studies limited confidence in a general conclusion.

In a later small trial, 61 people with diarrhoea-predominant IBS received a particular Bifidobacterium breve strain or a placebo for eight weeks. The trial found no effect on its main symptom-severity outcome. A finding about fatigue was exploratory and needs confirmation; some authors worked for a probiotics company. Neither result settles the effects of all strains or products.

Persistent or changing bowel symptoms deserve assessment rather than self-diagnosis. A clinician can consider your circumstances. For another product, see psyllium and irritable bowel symptoms.45

Safety and storage

Evidence about long-term safety is limited. Serious adverse effects may be more likely in people with severe underlying illness or weakened immune systems. Discuss use with a health professional when relevant. Follow the exact product’s storage instructions; see probiotic storage.1

When can live organisms be a safety concern?

Probiotics contain living organisms. In people who are critically ill, have weakened immune systems or have certain medical devices, an organism can sometimes cause an invasive infection.

A 2021 hospital-records study found Saccharomyces in the blood of 18 of 16,404 patients who had received a Saccharomyces boulardii yeast probiotic. This observational hospital study cannot give a risk for healthy people, every strain or every product. The count alone does not prove where each infection came from.

Preterm infants are a separate vulnerable group. In 2023, the US FDA warned of potentially fatal invasive infections associated with probiotic organisms in hospitalised preterm infants, including a reported infant death. Research on possible benefits and safety in neonatal care both need specialist consideration. Do not assume a retail product is appropriate for a baby; parents and caregivers should discuss the infant’s care with the neonatal team.67