TL;DR — Bottom Line
- Saccharomyces boulardii is a yeast, not a bacterium — antibiotics don't kill it, which is exactly why it's the probiotic most often recommended to take alongside (not just after) an antibiotic course.
- Florastor is the best overall pick — the original, most-studied branded S. boulardii formula, with the deepest clinical trial history of any product in this category.
- Klaire Labs Saccharomyces Boulardii is the clinical/practitioner-grade pick — hypoallergenic, higher per-capsule potency, favored by functional and integrative practitioners.
- Jarrow Formulas Saccharomyces Boulardii + MOS adds a prebiotic (mannan-oligosaccharides) that may support the yeast's binding activity, at a mid-range price.
- NOW Foods Saccharomyces Boulardii is the best budget option — same active organism, lower cost per capsule.
Quick Picks
Most probiotic advice defaults to bacterial strains — Lactobacillus, Bifidobacterium, various multi-strain blends — but there is one category that works by a fundamentally different mechanism and solves a specific, common problem: what to take during a course of antibiotics without the antibiotic itself destroying the probiotic. Saccharomyces boulardii is a probiotic yeast, and because antibiotics target bacteria, not fungi, it survives alongside the antibiotic in a way that Lactobacillus and Bifidobacterium products cannot.
This comparison looks at four widely available S. boulardii products by strain identity, potency, added ingredients, third-party testing, and evidence, specifically for antibiotic-associated diarrhea and related use cases.
Head-to-Head Comparison
| Feature | Florastor | Klaire Labs | Jarrow Formulas |
|---|---|---|---|
| Organism | Saccharomyces boulardii lyophilized (CNCM I-745) | Saccharomyces boulardii | Saccharomyces boulardii + MOS prebiotic |
| Potency per capsule | 250 mg | Higher-potency clinical dose | 5 billion CFU-equivalent |
| Added ingredients | Minimal, standardized formula | Hypoallergenic base, minimal excipients | Mannan-oligosaccharides (MOS) |
| Clinical trial history | Extensive — the strain used in most published RCTs | Same species, less brand-specific trial data | Same species, less brand-specific trial data |
| Price / month | $28–35 | $22–28 | $18–24 |
| Best for | General AAD prevention, most trial-backed choice | Sensitive individuals, practitioner-directed protocols | Those wanting added prebiotic support |
Why a Yeast Probiotic Works Differently Than Bacterial Strains
Antibiotic-associated diarrhea (AAD) affects an estimated 5–35% of people taking antibiotics, depending on the drug class and population studied, and it happens because antibiotics don't discriminate between the pathogen they're targeting and the beneficial bacteria living in the colon. Taking a bacterial probiotic (Lactobacillus, Bifidobacterium) during the antibiotic course runs into an obvious problem: the same antibiotic that's disrupting your native gut flora will often kill a meaningful portion of the supplemented bacteria too, depending on spectrum and timing.
The Two Things That Actually Differentiate S. Boulardii Products
- Strain/species identity — nearly all reputable products use the same species (S. boulardii), so this is less of a differentiator than with bacterial probiotics, where strain-specific trial data (like LGG or BB-12) matters enormously.
- Potency and formulation — differences show up more in dose per capsule, added prebiotics or excipients, and whether the specific branded strain (such as Florastor's CNCM I-745) has its own dedicated trial history versus being a generic version of the species.
This is a meaningful contrast with bacterial probiotics like Lactobacillus rhamnosus GG, where clinical evidence is tied to one specific, patented strain rather than the species as a whole — a distinction covered in more depth in GutCode's guide to probiotic strain specificity. With S. boulardii, most of the trial evidence has historically used the strain now marketed as Florastor, which is why it carries the deepest evidence base even though the underlying organism is broadly similar across brands.
Florastor Review
What it is
Florastor is the original branded Saccharomyces boulardii product, using the CNCM I-745 strain that has been studied in the majority of published randomized controlled trials on this organism, including hospital-based studies on antibiotic-associated diarrhea and C. difficile recurrence prevention.
The numbers
- 250 mg lyophilized (freeze-dried) S. boulardii per capsule, shelf-stable at room temperature
- Strain with the longest and most extensive published clinical trial history of any commercial S. boulardii product
- Available in capsule form, including a version designed to be opened and mixed with food or liquid for those with swallowing difficulty
- Widely used in hospital and physician-recommended protocols internationally
Who it's for
Florastor is the right call for anyone starting a course of antibiotics who wants the product with the deepest trial-specific evidence behind it, rather than a generic version of the species with less brand-specific data.
Affiliate disclosure: GutCode earns a commission on qualifying Amazon purchases at no additional cost to you.
Shop Florastor on AmazonKlaire Labs Saccharomyces Boulardii Review
What it is
Klaire Labs is a professional/practitioner supplement brand frequently recommended by integrative and functional medicine practitioners, and its Saccharomyces boulardii formula is built around a hypoallergenic base with minimal excipients, targeting people with multiple food or additive sensitivities.
What makes it different
The formulation strips out common allergens and fillers found in some consumer-grade probiotic capsules, which matters for people managing multiple sensitivities alongside a gut health issue (histamine intolerance, mast cell activation, multiple chemical sensitivity). It is often used as part of broader practitioner-directed gut restoration protocols rather than purchased as a standalone consumer item.
Who it's for
Klaire Labs is best suited to people with additional sensitivities who want a minimal-excipient formula, or anyone following a practitioner-guided protocol that specifies this brand.
Shop Klaire Labs Saccharomyces Boulardii on AmazonJarrow Formulas Saccharomyces Boulardii + MOS Review
What it is
Jarrow Formulas combines S. boulardii with mannan-oligosaccharides (MOS), a prebiotic fiber derived from yeast cell walls that some research suggests may help bind certain pathogenic bacteria (including some E. coli strains) in the gut, complementing the yeast's own mechanism.
The trade-offs
The added MOS is a reasonable rationale for combination formulas generally, though most of the direct clinical trial evidence for antibiotic-associated diarrhea prevention comes from S. boulardii alone, not the S. boulardii-plus-MOS combination specifically — so the added ingredient is a plausible bonus rather than a proven additive benefit backed by its own dedicated trials.
Who it's for
Jarrow's formula is a solid mid-range pick for people who want the core organism plus a reasonable-cost prebiotic addition, without paying practitioner-brand pricing.
Shop Jarrow Formulas Saccharomyces Boulardii + MOS on AmazonNOW Foods Saccharomyces Boulardii: The Budget Pick
NOW Foods offers a straightforward, no-frills S. boulardii capsule at one of the lowest price points among reputable brands, without the added prebiotic or hypoallergenic positioning of the other three products.
The numbers
- Saccharomyces boulardii, same active species as the premium options in this comparison
- Standard capsule format, no added MOS or specialty base
- Third-party testing consistent with NOW's broader manufacturing standards
- Lowest per-capsule cost of the four products reviewed here
What the research shows
Because the clinical evidence for antibiotic-associated diarrhea prevention (Szajewska & Kolodziej, 2015; McFarland, 2010, World Journal of Gastroenterology) is built around the species and dose rather than a single proprietary brand, a well-manufactured generic S. boulardii product is a reasonable evidence-aligned choice for people prioritizing cost, provided the label confirms an adequate CFU/mg dose consistent with studied ranges.
Who it's for
NOW Foods is the practical, budget-first pick for anyone who wants the core organism without paying for branding, added prebiotics, or a hypoallergenic base they don't specifically need.
Shop NOW Foods Saccharomyces Boulardii on AmazonWhich Saccharomyces Boulardii Product Should You Buy?
The right choice depends mostly on whether you want the most trial-specific evidence, a hypoallergenic formulation, an added prebiotic, or the lowest cost. Here is the decision matrix:
None of these products replace antibiotic therapy itself or a physician's guidance for recurrent C. difficile infection — S. boulardii is studied and used as an adjunct alongside standard treatment, not a substitute for it.
If you are immunocompromised, critically ill, or have a central venous catheter, review use of any S. boulardii product with your physician first, given the documented (though rare) risk of fungemia in these populations.