Saccharomyces boulardii explained: the probiotic that's a yeast

Not every probiotic is a bacterium. One of the best-known probiotics is a yeast called Saccharomyces boulardii, a relative of the yeast used to bake bread and brew beer.

Being a yeast gives it some unusual properties, some useful and some worth being careful about. Here's what it is, where the research is strongest and where it's thin.

What it is

Saccharomyces boulardii is considered a subtype of Saccharomyces cerevisiae, the species also known as baker's yeast or brewer's yeast (Enache-Angoulvant and Hennequin, 2005). In research and on labels, you'll often see it with the strain code CNCM I-745 (Maslennikov et al., 2026).

Because it's a fungus rather than a bacterium, it differs from Lactobacillus and Bifidobacterium strains in one important way. Antibiotics target bacteria, and they typically don't kill yeast (Preidis et al., 2020). That's a big part of why S. boulardii is so often discussed alongside antibiotics.

Where the evidence is strongest: antibiotic-associated diarrhea

A 2015 meta-analysis pooled 21 randomized trials with 4,780 people taking antibiotics. Compared with placebo or no intervention, S. boulardii was linked to a lower risk of antibiotic-associated diarrhea, from 18.7% down to 8.5%. Put another way, about 10 people needed to take it for one fewer person to get antibiotic-associated diarrhea. In adults specifically, the risk went from 17.4% to 8.2%, across 15 trials with 3,114 people (Szajewska and Kołodziej, 2015).

For the more serious C. difficile-associated diarrhea, the picture is less clear. In the same analysis, the reduction was significant in children, based on 2 trials, but not in adults, based on 9 trials with 1,441 people (Szajewska and Kołodziej, 2015).

The American Gastroenterological Association's technical review, which pooled trials of adults and children together, found S. boulardii was the only single-strain probiotic with a significant effect on C. difficile-associated diarrhea. It still rated the overall certainty of the evidence as low (Preidis et al., 2020). If you're considering it during a course of antibiotics, talk to your doctor or pharmacist about whether it fits your situation.

What about IBS and bloating?

Here the evidence is mixed. A strain-specific meta-analysis published in early 2026 looked at S. boulardii CNCM I-745 in IBS and described the results as conflicting. In two trials, people taking it were more likely to report a significant reduction in abdominal pain than those on placebo. But two other trials that measured symptom severity scores found no meaningful effect on abdominal pain, bloating or urgency (Maslennikov et al., 2026).

The authors called for more research. So if bloating is your main concern, S. boulardii doesn't currently have strong support for that specific goal.

The safety issue that's specific to yeast

In the IBS trials, side effects were no more common with S. boulardii than with placebo (Maslennikov et al., 2026). The rare but serious concern is fungemia, which means yeast in the bloodstream.

A review of 92 published cases of invasive Saccharomyces infection found that intravascular catheters and antibiotic treatment were the most common predisposing factors. S. boulardii accounted for 51.3% of the bloodstream infections, and the authors advised special caution with S. boulardii products (Enache-Angoulvant and Hennequin, 2005).

A later systematic review analyzed 108 cases of Saccharomyces fungemia reported between 2005 and 2022. About two-thirds (67.6%) occurred in patients who had received S. boulardii. Factors linked to those cases included a stay in intensive care, IV or tube feeding, existing diarrhea and diabetes (Vinayagamoorthy et al., 2023).

These infections are rare and mostly involve people who are already seriously ill. But they're the reason this probiotic isn't a casual choice for everyone.

Who should skip it or ask first

• People with a central venous catheter or other long-term IV line
• Anyone who is critically ill or in intensive care
• People with a weakened immune system
• People receiving IV or tube feeding
• Anyone taking antifungal medication, which could in theory affect a yeast-based probiotic. Ask your pharmacist.

And whatever you take, call your doctor if diarrhea during or after antibiotics is severe, bloody, or comes with fever or worsening abdominal pain.

If you and your doctor decide to try it

• Check the strain code. Look for the specific strain, such as CNCM I-745, rather than just "Saccharomyces boulardii," so you can match the product to the research.
• Be clear about the goal. The research is most consistent for diarrhea linked to antibiotics, not for everyday bloating.
• Follow the label on timing. Because antibiotics typically don't kill yeast, spacing doses away from an antibiotic is less of a concern than with bacterial probiotics (Preidis et al., 2020). Your pharmacist can confirm what makes sense with your medications.
• Keep it short and defined. Agree on how long you'll take it, rather than continuing indefinitely out of habit.

The bottom line

S. boulardii is a genuinely different kind of probiotic. It has its most consistent research for antibiotic-associated diarrhea, mixed results for IBS symptoms, and a rare but real safety concern for vulnerable patients. As with any probiotic, the strain, the reason you're taking it and your own health matter more than the promise on the front of the bottle.

Sources

1. Enache-Angoulvant A, Hennequin C. Invasive Saccharomyces infection: a comprehensive review. Clin Infect Dis. 2005;41(11):1559-68. PMID 16267727
2. Maslennikov R, Gosteeva E, Ananeva V, et al. Strain-specific systematic review with meta-analysis of probiotics efficacy in the treatment of irritable bowel syndrome. J Clin Med. 2026;15(3):1152. PMID 41682832
3. Preidis GA, Weizman AV, Kashyap PC, Morgan RL. AGA technical review on the role of probiotics in the management of gastrointestinal disorders. Gastroenterology. 2020;159(2):708-738.e4. PMID 32531292
4. Szajewska H, Kołodziej M. Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea. Aliment Pharmacol Ther. 2015;42(7):793-801. PMID 26216624
5. Vinayagamoorthy K, Pentapati KC, Prakash H. Epidemiology of Saccharomyces fungemia: a systematic review. Med Mycol. 2023;61(2):myad014. PMID 36806741

This article is for general information and isn't medical advice. Talk to your doctor or pharmacist before taking a yeast-based probiotic, especially if you have a central line, a weakened immune system or take antifungal or other prescription medication.