Benefits
Digestive support during antibiotic use
In a meta-analysis of 21 randomized trials (4,780 people), taking S. boulardii alongside antibiotics cut the share who developed antibiotic-associated diarrhea from 18.7% to 8.5%, with similar relative effects in children and adults. An earlier pooled analysis in adults found a comparable effect. Adult trials used about 200 to 1,000 mg a day during the antibiotic course.
Gut support during higher-risk antibiotic courses
A 2025 Cochrane review of 38 randomized trials (13,179 people taking antibiotics) found probiotics as a group lowered C. difficile-associated diarrhea from 3.2% to 1.6% (low-certainty evidence); in the S. boulardii trials the risk ratio was 0.48. The benefit came mostly from trials with unusually high baseline risk, and the two largest trials found no clear effect.
Shorter bouts of loose stools in children
In a meta-analysis of 23 randomized trials (3,450 children with acute gastroenteritis), S. boulardii shortened diarrhea by about one day and shortened hospital stays, but the evidence was rated very low quality because most trials were poorly blinded. A European pediatric gastroenterology working group gave it a weak recommendation.
Digestive comfort while traveling
In a meta-analysis of randomized trials in travelers, S. boulardii CNCM I-745 lowered the incidence of travelers' diarrhea by about 21% (risk ratio 0.79), the only one of three probiotics studied to reach significance. The largest trial, in about 3,000 Austrian travelers, found the effect depended on regular use and dose and varied by destination.
Fewer digestive side effects during H. pylori eradication therapy
Across 18 randomized trials (3,592 people), adding S. boulardii to standard antibiotic-based H. pylori therapy roughly halved overall side effects (risk ratio 0.47) and cut diarrhea (risk ratio 0.33), and slightly raised eradication rates (risk ratio 1.09). Evidence quality for the side-effect findings was rated low.
Mechanism of action
Not affected by antibacterial antibiotics
As a yeast, S. boulardii differs from bacterial probiotics in size, cell wall and antibiotic resistance, so antibacterial antibiotics do not kill it and it can be taken during an antibiotic course. Antifungal medicines are a different matter (see interactions).
Breaking down C. difficile toxins
In rat intestine and human colon tissue in the lab, S. boulardii releases a 54-kDa serine protease that digests C. difficile toxins A and B and their binding site on the gut lining, reversing much of the toxin damage to the tissue barrier.
Acting on microbes and on the gut lining
Animal and cell studies show two broad actions: on harmful microbes (binding bacteria, and enzymes such as a phosphatase that inactivates bacterial endotoxin) and on the gut lining itself (trophic, barrier-repair, anti-secretory and anti-inflammatory effects). In healthy people it does not change the existing gut microbiota.
Calming inflammatory signaling
Laboratory and animal work shows S. boulardii and proteins it secretes reduce pro-inflammatory cytokine production by interfering with NF-kappaB and the ERK1/2 and p38 MAP kinases, and raise PPAR-gamma, a receptor that helps protect the gut lining from inflammation.
Clinical trials
Systematic review and meta-analysis of randomized trials comparing S. boulardii with placebo or no treatment in children and adults taking antibiotics (Szajewska et al. 2015, Aliment Pharmacol Ther).
21 randomized trials, 4,780 children and adults taking antibiotics.
Antibiotic-associated diarrhea fell from 18.7% to 8.5% (risk ratio 0.47; 10 people treated for one to benefit), in children (risk ratio 0.43) and adults (risk ratio 0.49). C. difficile-associated diarrhea was reduced significantly in children (2 trials) but not in adults (9 trials, risk ratio 0.80, not significant).
Cochrane systematic review and meta-analysis of randomized trials of probiotics, including S. boulardii, given alongside antibiotics (Esmaeilinezhad et al. 2025, Cochrane Database Syst Rev).
47 randomized trials (15,260 adults and children taking antibiotics); 38 trials (13,179 people) reported C. difficile-associated diarrhea.
C. difficile-associated diarrhea occurred in 1.6% with probiotics versus 3.2% with control (risk ratio 0.50, low certainty), about one case prevented per 65 people treated. In the S. boulardii trials (2,565 people) the risk ratio was 0.48. The effect was large in trials with control-group risk above 5% (risk ratio 0.30) but small and not significant in the more typical trials below 3% (risk ratio 0.92). Adverse events were probably slightly fewer with probiotics.
Systematic review and meta-analysis with trial sequential analysis of randomized trials adding S. boulardii to standard H. pylori eradication therapy (Zhou et al. 2019, Helicobacter).
18 randomized trials, 3,592 patients.
Adding S. boulardii slightly raised eradication rates (risk ratio 1.09, moderate quality) and reduced total side effects (risk ratio 0.47) and diarrhea (risk ratio 0.33), both low quality evidence. Fewer people stopped therapy early (risk ratio 0.33).