What Is Saccharomyces Boulardii? Benefits and Side Effects

Saccharomyces boulardii is a tropical yeast used as a probiotic, most commonly taken to prevent or shorten bouts of diarrhea. Unlike the bacterial probiotics you may be more familiar with, it is a fungal organism, closely related to the common baker’s and brewer’s yeast found in bread and beer. It was first isolated from lychee and mangosteen fruit in Southeast Asia in the 1920s, and it has been commercially available as a supplement and over-the-counter remedy in many countries for decades. The research behind it is more substantial than for many probiotics, though, as with most things in gut health, the picture is messier than the supplement label suggests.

A Yeast, Not a Bacterium

Most probiotics on store shelves are bacteria, typically strains of Lactobacillus or Bifidobacterium. Saccharomyces boulardii stands apart because it is a yeast, a single-celled fungus. This distinction matters for practical reasons: because antibiotics kill bacteria but not fungi, S. boulardii can survive and function in your gut even while you are taking a course of antibiotics. That is the main reason it has attracted so much interest for antibiotic-related digestive problems.

Genetically, S. boulardii is extremely close to Saccharomyces cerevisiae, the yeast used in baking and brewing. In fact, genome comparisons have shown the two are so similar that many researchers now consider them the same species.¹ One genomic analysis placed S. boulardii strains alongside wine strains of S. cerevisiae on the family tree.² So the formal classification is often written as S. cerevisiae var. boulardii, meaning it is a variety of regular yeast rather than a wholly separate organism.

What sets it apart from the yeast in your bread is a handful of physical traits that make it suited for life in the human gut. Its optimal growth temperature matches human body temperature at 37°C, and it keeps the majority of its cells alive even after an hour at 52°C. It also survives passage through stomach acid and bile salts, which would destroy many other yeast strains.³ These traits are why swallowing a capsule of it can actually deliver living yeast to your intestines, whereas swallowing baker’s yeast would accomplish very little.

How It Works in the Gut

S. boulardii does not take up permanent residence in your intestines. Once you stop taking it, the yeast clears from the gut within a day or two.⁴ It is a transient visitor, not a colonizer. But during its time passing through, it does several things that researchers have been able to measure.

The best-understood mechanism involves a protease, a protein-digesting enzyme that S. boulardii secretes. This protease can break down toxins produced by Clostridium difficile, a dangerous gut pathogen. In lab studies, the protease reduced the binding of C. difficile toxin A to intestinal cells by about 42% and cut the intestinal damage caused by the toxin.⁵ Follow-up work showed the same protease also digests C. difficile toxin B and blocks both toxins from latching onto receptors on the lining of the human colon.⁶

Beyond toxin-fighting, S. boulardii appears to help keep the intestinal barrier intact. It interacts with immune signaling in the gut wall, dampening overactive inflammatory responses while making it harder for pathogens to adhere and colonize. The net effect is that the gut lining leaks less fluid into the intestines, which is essentially what stops diarrhea at a cellular level.⁷ These barrier-protective effects have been observed across several different gut conditions, not just C. difficile infection.⁸

There is also evidence that S. boulardii can reshape the bacterial community in the gut during antibiotic treatment. One study in mice found that adding the yeast during an antibiotic course increased bacterial species diversity and reduced the bloom of several bacterial families that antibiotics tend to promote, including Enterobacteriaceae and Enterococcaceae.⁴ In other words, the yeast may help the gut microbiome recover its balance faster after antibiotics disrupt it.

Preventing Antibiotic-Associated Diarrhea

The strongest clinical evidence for S. boulardii involves antibiotic-associated diarrhea, the loose, watery stools that many people develop while taking antibiotics. A meta-analysis pooling data from 21 randomized controlled trials found that taking S. boulardii roughly halved the risk: diarrhea occurred in about 8.5% of people taking the yeast alongside antibiotics, compared with about 19% of those on placebo. That held up in both adults and children.⁹

That said, the picture is not perfectly tidy. One well-designed randomized, double-blinded, placebo-controlled trial found no difference at all: diarrhea rates were essentially identical in the S. boulardii and placebo groups.¹⁰ Meanwhile, a different trial in hospitalized patients showed a dramatic effect, with diarrhea in about 8% of the S. boulardii group versus over 70% on placebo.¹¹ The wildly different results across individual trials likely reflect differences in patient populations, the type and duration of antibiotics used, dosing of the probiotic, and how researchers defined diarrhea. The overall pattern across the pooled data, though, leans in favor of some protective benefit.

C. Difficile Infection

C. difficile is the specific pathogen most often behind severe antibiotic-associated diarrhea, especially in hospitals and nursing homes. Because S. boulardii directly attacks C. difficile toxins, researchers have tested whether it can help prevent or treat this infection.

A randomized controlled trial found that adding S. boulardii to vancomycin, the standard antibiotic treatment for C. difficile, significantly reduced the chance of the infection coming back. Recurrence dropped from about 13% with vancomycin alone to under 2% with the combination.¹² A systematic review looking at earlier trials also found evidence of reduced recurrence, particularly in people who had already experienced repeated C. difficile episodes.¹³

Recurrence is the main headache with C. difficile. Roughly one in five patients who recover from an initial bout get sick again, and after a second episode the chances of a third climb higher. The fact that S. boulardii can be taken alongside the very antibiotic being used to fight the infection gives it an advantage that bacterial probiotics cannot match, since those would be killed by the drug. That said, most guidelines still consider S. boulardii a supplementary option, not a replacement for standard antibiotic treatment.

Children and Acute Diarrhea

Outside the antibiotic context, S. boulardii has been studied for acute gastroenteritis in children, the everyday stomach bugs that cause vomiting and diarrhea. A meta-analysis of randomized controlled trials found that children given S. boulardii had diarrhea that lasted about a day less than children given placebo.¹⁴ A separate meta-analysis confirmed the direction of the effect, finding a statistically meaningful reduction in diarrhea duration.¹⁵

A day less of diarrhea may not sound transformative, but for a sick child and their exhausted parents, it is clinically meaningful, especially in settings where dehydration is a serious risk. The evidence quality has been rated as low to very low in some reviews due to high variability between trials, so pediatricians may or may not recommend it depending on local guidelines. It is not a substitute for oral rehydration, which remains the cornerstone of managing childhood diarrhea.

Irritable Bowel Syndrome

IBS is a chronic condition, quite different from the acute infections discussed above, and the evidence for S. boulardii here is less mature. There are mechanistic reasons to think it could help: in lab models, S. boulardii shifts the balance of immune signaling molecules in the gut, lowering pro-inflammatory markers and raising anti-inflammatory ones.¹⁶ In mice with diarrhea-predominant IBS, the yeast slowed gut motility by affecting the serotonin transport system in the intestines.¹⁷

Those findings are interesting but mostly preclinical. Human trials on IBS tend to be small and produce mixed results across different probiotic strains. If you have IBS and are considering S. boulardii, it is reasonable to try, but go in with modest expectations. The evidence is far weaker here than it is for antibiotic-associated diarrhea.

Inflammatory Bowel Disease

Crohn’s disease and ulcerative colitis are autoimmune conditions affecting the gut, and a few small trials have looked at S. boulardii as an add-on therapy. In one pilot study of ulcerative colitis, 17 out of 24 patients who completed the trial reached clinical remission, confirmed by endoscopy, while taking S. boulardii alongside standard treatment.¹⁸ In Crohn’s disease, a small trial of 32 patients in remission found that those receiving S. boulardii plus mesalamine had a relapse rate of about 6%, compared with about 38% in those on mesalamine alone over six months.¹⁹

These numbers look impressive, but both studies were small and lacked the rigor of large, multi-center trials. They are best understood as signals worth pursuing, not proof. No major gastroenterology guideline currently recommends S. boulardii as a standard part of IBD treatment, though individual doctors sometimes suggest it as a low-risk addition.

Side Effects and Safety Risks

For most people, S. boulardii is well tolerated. The most commonly reported side effects are mild: gas, bloating, and occasional thirst. Serious reactions in healthy individuals are extremely rare in the published literature.

The real safety concern involves a specific and uncommon complication: fungemia, meaning the yeast enters the bloodstream. This has been reported almost exclusively in people who are severely ill, immunocompromised, or who have central venous catheters. One case series described seven instances in a single intensive care unit over two years, all in critically ill patients on mechanical ventilation with central lines.²⁰ A case report documented fungemia in a 79-year-old woman with rheumatoid arthritis after bowel surgery; she was successfully treated with antifungal medication, but the episode was serious enough to require stopping the probiotic immediately.²¹

One wrinkle worth knowing: fungemia has even been reported in patients who were not personally taking S. boulardii but were in the same hospital room or ICU as someone who was. The theory is that opening capsules of the yeast can aerosolize spores, which then contaminate nearby central venous catheters.²² This is why many hospitals have policies restricting the use of S. boulardii near patients with central lines, regardless of whether the patient themselves is the one taking it.

The practical takeaway: if you are generally healthy and taking S. boulardii orally for a short course of antibiotics or a bout of diarrhea, the safety profile is favorable. If you are immunocompromised, critically ill, or have a central venous catheter, the risk calculus shifts dramatically. This is a supplement that should genuinely be discussed with a physician in those situations.

How It Passes Through You

S. boulardii does not colonize the gut permanently, which is part of what makes it safe but also part of what limits its usefulness. After you stop taking it, the yeast is undetectable in stool within a few days.⁴ This means its effects last only as long as you keep taking it. For something like antibiotic-associated diarrhea prevention, the standard approach is to start the yeast when you start the antibiotic and continue for a few days after completing the course.

Most supplement labels recommend doses in the range of 250 to 500 mg per day, typically taken as one or two capsules. Some clinical trials have used higher doses, but there is no strong consensus on the optimal amount. The yeast is available as capsules, sachets, and chewable tablets depending on the brand and country. Because it is sold as a dietary supplement in the United States rather than a drug, formulations are not regulated as tightly as pharmaceuticals, and the viable cell count in a given capsule may vary between manufacturers.

Combining S. Boulardii with Prebiotics

A growing area of research asks whether pairing S. boulardii with prebiotics, fibers and compounds that feed beneficial microbes, could boost the yeast’s effectiveness. A recent narrative review found that various types of prebiotics at different concentrations were able to enhance both the viability of S. boulardii and its functional effects on the gut environment.²³ Some supplement products already combine the yeast with prebiotic fibers for this reason, marketed as “synbiotic” formulations. Whether those combinations outperform the yeast alone in human clinical trials is still being worked out.

Where It Came From

The yeast’s name comes from Henri Boulard, a French scientist who isolated it in 1920 during a cholera outbreak in Indochina (present-day Southeast Asia). Boulard noticed that some locals who chewed the skin of lychee and mangosteen fruits did not develop cholera symptoms, which led him to investigate the microorganisms on the fruit skins. He eventually isolated the yeast strain that now bears his name. It was commercialized in France beginning in the 1950s and has since spread to over 100 countries. The most widely studied strain in clinical trials is designated CNCM I-745 (also sold under the brand name Florastor in the United States and Ultra-Levure in France and other European markets).

The origin story is one of those appealing narratives that may be somewhat simplified by time, but the core facts, a tropical yeast isolated from fruit skin with observable protective effects against gut infection, have been confirmed by modern genomics and microbiology. What Boulard observed empirically a century ago, researchers can now explain at the level of enzymes, immune signals, and barrier integrity.