Almost every common probiotic, Lactobacillus and Bifidobacterium strains especially, is a bacterium. Saccharomyces boulardii is a yeast, related to baker's yeast but a distinct strain, and that difference has a practical consequence: it survives antibiotics that would otherwise kill bacterial probiotics taken at the same time.
A 2010 meta-analysis reviewed 27 randomized controlled trials covering more than 5,000 adult patients and found S. boulardii effective and safe in 84% of the treatment comparisons studied.[1] The strongest evidence was for preventing antibiotic-associated diarrhea, pooled across 10 trials with a relative risk of 0.47, roughly half the risk compared with not taking it. The number needed to treat was about 10, meaning treating 10 people prevents one case.
It also showed a specific benefit alongside vancomycin for recurrent C. difficile infection: 16.7% recurrence with S. boulardii added, versus 50% on high-dose vancomycin alone.[1] For H. pylori treatment, it didn't improve eradication rates directly. It did cut the diarrhea and side effects that often cause people to stop their antibiotic course early, 14.5% versus 30.6% in the comparison trials.
What to do with this
If you're starting a course of antibiotics and want to reduce your risk of antibiotic-associated diarrhea, S. boulardii has the strongest trial support among probiotic options. The number needed to treat is around 10. The typical dose is 250 to 500 milligrams twice daily, starting alongside the antibiotic. Take it several hours apart from the antibiotic dose itself if your product recommends spacing, and continue for a few days after finishing the antibiotic course. Skip it if you're severely immunocompromised or have a central intravenous line, and check with your doctor first in that situation.
