Last updated: April 2026
Saccharomyces boulardii is one of the most studied probiotic organisms in existence, and one of the most misunderstood.
It is not a bacterium. It is a yeast, and that distinction matters. Unlike bacterial probiotics, S. boulardii is naturally resistant to antibiotics, which is precisely why it is useful during antibiotic treatment. It survives where most other probiotics cannot.
Here is what the research actually shows about how it works, what it helps with, where the evidence is strong, and where it is not.
What Is Saccharomyces boulardii?
Saccharomyces boulardii is a non-pathogenic yeast originally isolated from lychee and mangosteen fruit in Southeast Asia in the 1920s by French scientist Henri Boulard. It was later developed into a probiotic after Boulard noticed that local populations used the fruit’s skin to manage cholera-related diarrhoea.
The strain you will see cited in almost every clinical trial is S. boulardii CNCM I-745. This is the reference strain, and most of the evidence in this article comes from studies using it.
It is closely related to Saccharomyces cerevisiae (baker’s yeast) but behaves differently in the gut microbiome. S. boulardii is a transient organism: it does not permanently colonise the gut, but it exerts significant effects during the time it is present.
Its most important property is antibiotic resistance. Antibiotics target bacteria, not yeast, which means S. boulardii remains active and effective even when taken alongside a course of antibiotics, something bacterial probiotics cannot claim.
How S. boulardii Works in the Gut
Once swallowed, S. boulardii survives stomach acid and reaches the intestines largely intact, an advantage over many bacterial strains that are partially degraded in transit.
In the gut, it operates through several mechanisms:
- Pathogen competition: It competes with harmful bacteria for attachment sites on the intestinal lining and can neutralise toxins produced by pathogens such as Clostridioides difficile.
- Barrier support: It strengthens the intestinal barrier and stimulates the production of secretory immunoglobulin A (sIgA), an antibody that acts as the gut’s first line of immune defence.
- Trophic effects: It supports enterocyte (intestinal cell) maturation and brush-border enzyme activity, which are essential for normal digestion and absorption.
- Anti-inflammatory signalling: Laboratory research shows it can dampen inflammatory pathways involving NF-κB, IL-6 and TNF-α, reducing inflammation without broadly suppressing immune function.
- Microbiome modulation: It supports the recovery of beneficial bacterial populations after disruption, which is particularly relevant in the context of antibiotics.
Benefits of Saccharomyces boulardii
Antibiotic-associated diarrhoea (AAD)
This is the strongest evidence base for S. boulardii. Antibiotics disrupt the gut microbiome, often causing diarrhoea as collateral damage. Because S. boulardii is a yeast and therefore unaffected by antibiotics, it can be taken concurrently to help maintain gut stability.
A systematic review and meta-analysis of 27 randomised controlled trials involving 5,029 patients found that S. boulardii reduced the risk of antibiotic-associated diarrhoea with a relative risk of 0.47 (95% CI 0.35–0.63, p<0.001), meaning roughly a 53% lower risk compared with placebo or no treatment. The effect was consistent across both adult and paediatric populations (McFarland, 2010).
An earlier meta-analysis published in Alimentary Pharmacology & Therapeutics (Szajewska & Kolodziej, 2015) of 21 randomised controlled trials reached similar conclusions. By probiotic standards, the evidence here is robust: multiple independent trials, a consistent direction of effect, and a clear biological rationale.
Traveller’s diarrhoea
Traveller’s diarrhoea is typically caused by exposure to unfamiliar bacteria in food and water, particularly E. coli strains. S. boulardii has been studied as a way to reduce the risk, with trials showing a modest but meaningful reduction in incidence when taken before and during travel.
The evidence is less definitive than for antibiotic-associated diarrhoea, but S. boulardii is among the better-studied options for this purpose.
H. pylori support
Helicobacter pylori is a bacterium that colonises the stomach lining and is associated with gastric ulcers and chronic gastritis. Standard treatment involves a course of antibiotics (known as eradication therapy), which can be harsh on the gut.
Several clinical trials have found that adding S. boulardii to H. pylori eradication therapy reduces the gastrointestinal side effects of treatment, particularly nausea and diarrhoea, without interfering with antibiotic efficacy. A meta-analysis in the World Journal of Gastroenterology (Szajewska et al., 2015) confirmed this effect across multiple trials.
Clostridioides difficile (C. diff): what the evidence does and does not support
Clostridioides difficile (previously Clostridium difficile, still widely known as C. diff) is a serious bacterial infection of the colon that typically occurs after a course of antibiotics has disrupted the normal gut flora. It causes severe diarrhoea and, in vulnerable patients, can be life-threatening.
If you have suspected or confirmed C. diff, this is a medical condition that requires NHS or GP care. S. boulardii is not a treatment for C. diff, and it is not a substitute for antibiotics or medical assessment. Anyone with persistent diarrhoea after antibiotic use, particularly with fever, severe abdominal pain or blood in the stool, should contact their GP or NHS 111 promptly.
With that out of the way, the research question people are actually asking is whether S. boulardii has any role alongside standard treatment. The honest answer is: the evidence is mixed, and the benefit (where it exists) is narrow.
Two older randomised controlled trials remain the most-cited positive data:
- McFarland et al. (1994) found that, in patients with recurrent C. diff, S. boulardii taken alongside standard antibiotics reduced recurrence to 34.6%, compared with 64.7% on placebo (p=0.04).
- Surawicz et al. (2000) found that, when combined specifically with high-dose vancomycin in recurrent C. diff, S. boulardii reduced recurrence to 16.7%, compared with 50% on placebo (p=0.05). The benefit was not seen with lower-dose vancomycin or with metronidazole.
More recent evidence complicates the picture. A 2022 systematic review and meta-analysis covering eight studies concluded that, when pooled, S. boulardii showed no clear benefit for preventing either primary or recurrent C. diff infection in patients taking systemic antibiotics (Mosholder et al., 2022). A 2025 randomised controlled trial of 120 patients reported that adding S. boulardii to vancomycin reduced recurrence in mild-to-moderate C. diff without affecting recovery or increasing adverse events, which brings some fresh positive data to a previously uncertain field.
Taking all of this together, the most honest reading of the evidence is this:
- S. boulardii is not supported for preventing a first episode of C. diff. If you are worried about developing C. diff from an antibiotic course, the relevant evidence is for reducing antibiotic-associated diarrhoea generally, not for preventing C. diff specifically.
- There is limited, inconsistent evidence that it may help as an adjunct to standard antibiotic treatment in people with recurrent C. diff. This is a narrow clinical use, supervised by a medical team, and is not the same as self-supplementing.
- It is not a treatment for C. diff on its own and should never be used as a substitute for medical care.
If you have been given a C. diff diagnosis, any decision about probiotics should be discussed with your GP or the clinical team managing your treatment.
Irritable bowel syndrome (IBS)
S. boulardii has been studied in IBS, with mixed but generally positive results. Its effects on intestinal inflammation and barrier function are the most plausible mechanisms.
A study published in Digestive Diseases and Sciences (Choi et al., 2011) found that S. boulardii improved overall IBS symptoms and quality of life compared with placebo. The evidence is not as consistent as for antibiotic-associated diarrhoea, but it remains a reasonable option for those who want to trial a live culture supplement.
Small intestinal bacterial overgrowth (SIBO)
SIBO occurs when bacteria from the colon proliferate abnormally in the small intestine. Standard treatment involves antibiotics, but recurrence rates are high and the disruption to the broader microbiome can compound the problem.
S. boulardii has attracted attention in SIBO management for a specific reason: as a yeast, it does not contribute to bacterial overgrowth. Bacterial probiotic supplements can theoretically worsen SIBO if the wrong strains are used, a concern that does not apply to S. boulardii.
Clinical evidence remains limited, with most data coming from small trials rather than large randomised controlled trials. What exists suggests S. boulardii may help reduce symptoms and support microbiome stability during and after antibiotic treatment for SIBO. It is not a primary treatment, and anyone with suspected SIBO should be assessed by a healthcare professional before supplementing.
Candida and thrush
There is some evidence that S. boulardii may help suppress Candida overgrowth in the gut. It competes with Candida albicans for space and nutrients and may inhibit its adhesion to the intestinal lining.
The evidence base here is smaller and less definitive than for diarrhoea conditions. S. boulardii should not be used as a standalone treatment for systemic Candida infections, and anyone with a diagnosed Candida overgrowth should seek medical advice. As a supportive measure alongside appropriate treatment, it may offer some benefit.
Post-antibiotic microbiome recovery
Antibiotics can leave the gut microbiome disrupted for weeks or months after the course ends, and the question of how best to support recovery is an active area of research.
A 2018 study in Cell (Suez et al.) found that taking standard bacterial probiotic supplements immediately after antibiotics actually delayed microbiome recovery compared with no supplementation. That finding applied to bacterial probiotics only; S. boulardii was not tested in that study.
S. boulardii has been tested directly in a 2023 study published in Frontiers in Medicine (Barbut et al., 2023), using a human microbiota-associated mouse model (mice colonised with human gut bacteria, used as a translational proxy before human trials). In that preclinical model, animals that received S. boulardii during antibiotic treatment showed better recovery of their bacterial populations, and extending supplementation for a further seven days after the antibiotic course ended supported a fuller return towards the baseline microbial equilibrium. Effects on the fungal component of the microbiome were limited once the yeast itself had cleared.
This is mechanistically encouraging, but it is a preclinical finding rather than a human trial, and the often-quoted “17-day optimal course” comes from that mouse model. It does not yet constitute a proven protocol for humans. What it does suggest, cautiously, is that continuing S. boulardii for a period beyond the antibiotic course may be more useful than stopping the same day as the antibiotic, which is consistent with how most clinical AAD trials have dosed it.
Dosage and How to Take It
Most clinical trials of S. boulardii have used doses in the following ranges:
| Use case | Typical dose | Duration in trials |
|---|---|---|
| Antibiotic-associated diarrhoea | 250–500 mg, twice daily | During antibiotic course + at least 1–2 weeks after |
| Traveller’s diarrhoea | 250–500 mg daily | Starting 3–5 days before travel, throughout trip |
| General gut support | 250–500 mg daily | Ongoing, at a consistent time each day |
| Acute protocols (clinical use) | Up to 1 g/day, divided | Short-term, under clinical supervision |
As with most supplements, starting at a lower dose to assess tolerance before increasing is sensible. S. boulardii can be taken with or without food. Unlike most bacterial probiotics, it does not require refrigeration: the yeast is heat-stable, though storage conditions vary by product.
When to Take S. boulardii
There is no single best time of day to take S. boulardii. Consistency matters more than the clock: taking it at the same time each day helps maintain steady levels in the gut, as it clears the system within a few days of stopping.
Timing guidance depends on the reason for use:
- During antibiotic treatment: Start on the same day as the antibiotic and continue for at least two weeks after completing the course. Unlike bacterial probiotics, S. boulardii does not need to be taken at a different time from the antibiotic, as it is antibiotic-resistant by nature.
- For traveller’s diarrhoea: Begin 3–5 days before travel and continue throughout the trip.
- For general gut support: Morning or evening, with or without food. Any routine you can maintain consistently is the right one.
Side Effects and Safety
S. boulardii is generally well tolerated. The most commonly reported side effects are mild and transient: bloating and gas as the gut adjusts, particularly in the first few days.
There are two groups for whom caution is warranted. People who are severely immunocompromised, including those on immunosuppressive therapy or with central venous catheters, should avoid S. boulardii and all live probiotic supplements without first consulting their doctor. There is a small but documented risk of fungaemia (yeast entering the bloodstream) in this population.
Anyone with a confirmed yeast allergy should also avoid S. boulardii.
For healthy adults, the safety record across decades of clinical use is reassuring, and S. boulardii CNCM I-745 holds Generally Recognised As Safe (GRAS) status in food applications.
What to Look For in an S. boulardii Supplement
- Strain identified: The product should list Saccharomyces boulardii as an active ingredient, ideally with the strain designation. CNCM I-745 is the most clinically studied.
- Live cultures confirmed: Check that the product guarantees viable organisms at time of use, not just at manufacture.
- No unnecessary additives: Fillers, artificial sweeteners and synthetic binders add nothing and may counteract the gut health benefit.
- Transparent labelling: The CFU (colony-forming units) count should be stated clearly.
S. boulardii is one of the six strains in Biome Bliss, our naturally fermented gut health supplement. For a broader overview of the strains in Biome Bliss and what each one does, see our guide to probiotic strains in Biome Bliss.
For wider context on how the gut ecosystem works and why it matters, see our introduction to the gut microbiome, and our guide to the microbiome diet for practical ways to support it through food. For a closer look at one of the most studied bacterial probiotics often used alongside S. boulardii, see our guide to Lactobacillus rhamnosus. For context on how live cultures fit alongside prebiotics and postbiotics, see our guide to the differences between prebiotics, probiotics and postbiotics.
Frequently Asked Questions
What is Saccharomyces boulardii used for?
S. boulardii is most commonly used to help reduce the risk of antibiotic-associated diarrhoea, to support gut stability during travel, and as an adjunct alongside H. pylori eradication therapy. It has also been studied in IBS. It is a yeast-based live culture, making it unique in its ability to remain active during antibiotic treatment.
Can I take S. boulardii with antibiotics?
Yes, and this is one of its primary advantages. Because it is a yeast rather than a bacterium, antibiotics do not affect it. Taking S. boulardii alongside antibiotics may help reduce the risk of antibiotic-associated diarrhoea. Most clinical trials continue supplementation for at least one to two weeks after the antibiotic course ends.
Does S. boulardii help with C. difficile?
The evidence is mixed and narrow. S. boulardii is not a treatment for C. diff and is not supported for preventing a first infection. Limited research suggests it may reduce recurrence in people with recurrent C. diff when combined with standard antibiotics such as high-dose vancomycin, but a 2022 pooled analysis found no clear benefit overall. Anyone with suspected C. diff should contact their GP or NHS 111.
What are the side effects of Saccharomyces boulardii?
Side effects are generally mild. Some people experience bloating or gas when first starting, which typically resolves within a few days. Immunocompromised individuals should consult a doctor before use, as there is a small risk of fungaemia in this population.
How long does it take for S. boulardii to work?
For acute conditions such as antibiotic-associated diarrhoea, effects can be noticeable within a few days. For ongoing gut health support, most trials use intervention periods of two to four weeks before assessing outcomes.
Is S. boulardii good for Candida?
There is some evidence that S. boulardii may inhibit Candida overgrowth in the gut by competing for attachment sites. The evidence is less robust than for diarrhoea applications. Anyone with a diagnosed Candida infection should seek medical advice rather than relying on supplementation alone.
Does S. boulardii help with constipation?
S. boulardii is primarily studied for diarrhoeal conditions, where the evidence is strongest. Evidence for constipation is limited. Most trials focus on diarrhoea-predominant IBS or acute diarrhoea, and constipation is not a primary indication.
Can I take S. boulardii with other probiotics?
Yes. S. boulardii works through different mechanisms than bacterial strains and is generally considered complementary rather than competitive. It is commonly combined with Lactobacillus or Bifidobacterium strains in clinical practice and multi-strain formulations. One of the most studied bacterial partners is Lactobacillus rhamnosus — see our overview of Lactobacillus rhamnosus for more on where the evidence is strongest. A 2024 laboratory study in The ISME Journal found that, in defined in vitro microbial communities, adding S. boulardii increased the production of anti-inflammatory metabolites and supported the growth of beneficial bacterial strains. This is a preclinical finding, not a human trial, but it is consistent with the long-standing clinical practice of combining the two.
Is S. boulardii safe for people with histamine intolerance?
S. boulardii is not known to produce histamine, unlike certain fermented foods and some bacterial strains. For most people with histamine sensitivity, it is generally well tolerated. If you are highly reactive to yeast-based or fermented products, start with a low dose and monitor your response. Consult a healthcare professional if in doubt.
References
- McFarland LV (2010). Systematic review and meta-analysis of Saccharomyces boulardii in adult patients. World Journal of Gastroenterology, 16(18), 2202–2222. pubmed.ncbi.nlm.nih.gov/20458757
- Szajewska H & Kołodziej M (2015). Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea. Alimentary Pharmacology & Therapeutics, 42(7), 793–801. pubmed.ncbi.nlm.nih.gov/26216624
- Szajewska H et al. (2015). Saccharomyces boulardii supplementation and eradication of Helicobacter pylori infection: a systematic review and meta-analysis. World Journal of Gastroenterology, 21(15), 4661–4672. pubmed.ncbi.nlm.nih.gov/25914477
- Choi CH et al. (2011). A randomized, double-blind, placebo-controlled multicenter trial of Saccharomyces boulardii in irritable bowel syndrome. Digestive Diseases and Sciences. pubmed.ncbi.nlm.nih.gov/21560040
- McFarland LV et al. (1994). A randomized placebo-controlled trial of Saccharomyces boulardii in combination with standard antibiotics for Clostridium difficile disease. JAMA, 271(24), 1913–1918. pubmed.ncbi.nlm.nih.gov/8201735
- Surawicz CM et al. (2000). The search for a better treatment for recurrent Clostridium difficile disease: use of high-dose vancomycin combined with Saccharomyces boulardii. Clinical Infectious Diseases, 31(4), 1012–1017. pubmed.ncbi.nlm.nih.gov/11049785
- Mosholder AD et al. (2022). Systematic review and meta-analysis: safety and efficacy of Saccharomyces boulardii for the prevention of Clostridioides difficile infection. Therapeutic Advances in Gastroenterology. scholar.rochesterregional.org/rrhpubs/1702
- Barbut F et al. (2023). Saccharomyces boulardii CNCM I-745 supplementation during and after antibiotic treatment positively influences the bacterial gut microbiota. Frontiers in Medicine. Preclinical (human microbiota-associated mouse model). pubmed.ncbi.nlm.nih.gov/37601783
- Suez J et al. (2018). Post-antibiotic gut mucosal microbiome reconstitution is impaired by probiotics and improved by autologous FMT. Cell, 174(6), 1406–1423. pubmed.ncbi.nlm.nih.gov/30193113
- Gu Y et al. (2024). Saccharomyces boulardii enhances anti-inflammatory effectors and AhR activation via metabolic interactions in probiotic communities. The ISME Journal, 18(1), wrae212. In vitro defined-community study. pmc.ncbi.nlm.nih.gov/articles/PMC11631509
This article is for informational purposes only and does not constitute medical advice. If you are experiencing digestive symptoms, or if you have been treated for C. difficile, please consult a GP or registered healthcare professional.
