Saccharomyces boulardii is a tropical yeast used as a probiotic to prevent and treat diarrhea, particularly the kind caused by antibiotics. Unlike the bacterial probiotics you’ll find in yogurt or fermented foods, this is a fungal microorganism, closely related to common baker’s yeast. It’s one of the most studied probiotics in the world, with decades of clinical trial data behind it, and it’s sold over the counter in capsule and powder form under several brand names.
A Yeast, Not a Bacterium
Most probiotics are bacteria, typically strains of Lactobacillus or Bifidobacterium. Saccharomyces boulardii stands apart because it’s a yeast, a single-celled fungus. It was originally isolated from tropical fruit in Southeast Asia and is technically a variant of Saccharomyces cerevisiae, the same species used to brew beer and bake bread. But it behaves differently in your body: it thrives at 37°C (body temperature), survives stomach acid and bile, and isn’t killed by antibiotics. That last point matters a lot, because it means you can take it alongside antibiotics without the medication wiping it out the way it would wipe out bacterial probiotics.
Once you stop taking it, the yeast doesn’t permanently colonize your gut. It passes through in a few days. This transient nature is part of why it has such a strong safety profile for most people.
How It Works in the Gut
Saccharomyces boulardii protects your digestive tract through several overlapping mechanisms, which is part of why it’s useful for such a range of gut problems.
The yeast releases an enzyme that physically breaks down toxins produced by harmful bacteria. For example, it cleaves the toxin made by C. difficile (a dangerous bacterium that causes severe diarrhea, often after antibiotic use) and also degrades the receptor that toxin latches onto. It produces another enzyme that neutralizes toxins from E. coli by chemically deactivating them, which reduces the inflammatory cascade those toxins trigger.
Beyond toxin neutralization, the yeast acts as a decoy. Harmful bacteria like certain strains of E. coli bind to the surface of S. boulardii instead of attaching to your intestinal lining. It also releases compounds that interfere with the ability of dangerous microbes to stick to gut cells and reduces overgrowth of Candida albicans, a common opportunistic fungus.
At the level of the gut lining itself, S. boulardii strengthens the barrier between your intestinal cells. It helps maintain the tight junctions that keep your gut wall sealed, delays cell death in intestinal tissue under stress, and activates a receptor called PPAR-gamma that dials down inflammation. In animal studies, it substantially reduced markers of gut inflammation including several key inflammatory signaling molecules.
Preventing Antibiotic-Associated Diarrhea
This is the best-supported use of S. boulardii. A meta-analysis of 21 randomized controlled trials covering 4,780 participants found that taking the yeast during antibiotic treatment cut the risk of diarrhea roughly in half. In the placebo groups, about 18.7% of people developed diarrhea while on antibiotics. In the groups taking S. boulardii, that dropped to 8.5%. The results held for both adults and children, with children seeing a slightly larger benefit (risk dropped from 20.9% to 8.8%).
For every 10 people who take S. boulardii alongside antibiotics, one person is spared a bout of diarrhea they would have otherwise developed. That’s a meaningful effect for something available without a prescription and with minimal side effects.
C. Difficile Infection and Recurrence
C. difficile infections are notoriously hard to shake. Even after successful treatment, the infection comes back in a significant percentage of patients. A randomized controlled trial of 120 patients tested S. boulardii as an add-on to standard antibiotic treatment. Patients received 500 mg of S. boulardii daily (two 250 mg capsules) alongside their prescribed antibiotic for 10 days.
The recurrence rate in the combination group was 1.7%, compared to 13.1% in the group receiving the antibiotic alone. The overall cure rate (meaning the infection resolved and stayed gone) was about 97% in the combination group versus 85% with the antibiotic alone. Initial clinical cure rates were nearly identical between groups, suggesting S. boulardii’s main contribution is preventing the infection from bouncing back rather than treating the acute episode.
Effects on Irritable Bowel Syndrome
The evidence for IBS is more modest. A double-blind, placebo-controlled trial in patients with diarrhea-predominant or mixed-type IBS found that S. boulardii improved quality of life more than placebo (15.4% improvement versus 7.0%). All eight measured quality-of-life domains improved in the treatment group, while only two improved in the placebo group. However, the yeast did not outperform placebo on individual symptoms like bowel frequency or stool consistency. So while people felt better overall, the measurable gut symptoms didn’t change dramatically. For IBS, S. boulardii may help, but it’s not a reliable fix.
Typical Dosing
Most clinical trials use 250 to 500 mg per day, often split into two doses. In capsule form, this typically translates to 5 to 10 billion colony-forming units (CFUs) daily. Some trials have used higher doses (up to 750 mg or 1,000 mg daily) for conditions like IBS, and some pediatric studies have used as little as 250 mg once daily.
For antibiotic-associated diarrhea prevention, the most common studied regimen is 250 mg twice daily, started at the beginning of the antibiotic course and continued for its duration. Some protocols extend supplementation a few days beyond the last antibiotic dose. The yeast is available as capsules, sachets of powder that can be mixed into food or drink, and chewable tablets.
Safety and Who Should Avoid It
For the vast majority of people, S. boulardii is well tolerated. Side effects in clinical trials are rare and generally limited to gas or mild bloating. But there is one serious risk that applies to a small, specific population.
A CDC review identified 46 patients who developed Saccharomyces fungemia, meaning the yeast entered their bloodstream. At least 43% of those cases were in people taking S. boulardii as a probiotic. The odds of this happening were 14 times higher in people using the probiotic compared to a control group. Nearly all reported cases involved people who were critically ill, severely immunocompromised, or had central venous catheters (IV lines placed in large veins). In some cases, the fungemia appeared to result from the yeast contaminating a catheter rather than crossing through the gut.
Clinical trials have not observed fungemia in their participants, but those trials typically exclude the sickest patients. If you have a weakened immune system, are in intensive care, or have a central line in place, S. boulardii is not appropriate. For otherwise healthy people or those simply looking to prevent antibiotic side effects, the safety record across thousands of trial participants is strong.