What happens when you take saccharomyces boulardii with antifungals?
Saccharomyces boulardii is a live yeast probiotic, sold under brand names such as Florastor and used for traveler's diarrhea, antibiotic-associated diarrhea, and recurrent Clostridioides difficile infection. Despite being marketed as a probiotic, it is biologically a yeast and is closely related to baker's yeast, Saccharomyces cerevisiae.
Systemic antifungal medications are designed to kill yeasts. The azole class, including fluconazole, itraconazole, voriconazole, and posaconazole, inhibits an enzyme essential to yeast cell-membrane synthesis. The polyene class, including amphotericin B and nystatin, punches holes in yeast membranes. The echinocandin class, including caspofungin, micafungin, and anidulafungin, disrupts yeast cell-wall construction. All three classes are active against S. boulardii as well as against pathogenic Candida.
Co-administering the two is therefore self-defeating. The antifungal kills the probiotic, so the patient receives no probiotic benefit. The patient still incurs the cost and any swallowing burden of the supplement.
Why is this important?
Beyond simple loss of probiotic effect, there is a more serious concern. The CDC, the Emerging Infectious Diseases journal, and multiple case series have documented S. boulardii fungemia, a bloodstream infection caused by the probiotic organism itself. The risk is highest in patients in intensive care units, patients with central venous catheters, immunocompromised patients, and preterm infants. In these cases, the same antifungals that would have been killing the probiotic if co-administered are instead being used as treatment.
A patient who is on antifungal therapy is almost by definition in a situation where opportunistic yeast infection is a real concern. Deliberately introducing more yeast into the gut during this period adds risk without benefit. Once the antifungal stops, the residual S. boulardii may re-grow and, if it crosses a damaged gut wall, can cause invasive disease.
The published case literature is clear enough that hospital pharmacies frequently restrict S. boulardii use in critically ill patients and in patients with central lines, even when an antifungal is on board.
What should you do?
If you have been prescribed a systemic antifungal, stop taking Saccharomyces boulardii products such as Florastor immediately and tell the prescribing physician. Ask whether a bacterial probiotic could substitute if a probiotic is still desired. Bacterial probiotics including Lactobacillus and Bifidobacterium species are not directly killed by azole antifungals.
If you are admitted to a hospital and a clinician asks about home supplements, mention any probiotic explicitly. Florastor capsules look unremarkable and are easy to overlook, but they are clinically meaningful. If you have a central venous catheter, port, or PICC line, do not handle S. boulardii capsules near the catheter site because contamination has been linked to fungemia.
Topical antifungals such as clotrimazole or terbinafine cream do not have meaningful systemic exposure and do not interact with oral S. boulardii. Oral nystatin used as a swish-and-spit for thrush has minimal systemic absorption but can still reduce S. boulardii viability in the upper gut. Ask your prescriber.
Which specific products are affected?
Products of concern include Florastor, Florastor Kids, DiaResQ, and any supplement listing Saccharomyces boulardii, S. cerevisiae var. boulardii, or generic Saccharomyces cerevisiae. Some multi-strain probiotic blends include S. boulardii alongside bacterial species, so read the ingredient panel carefully.
Antifungals of concern include fluconazole (Diflucan), itraconazole (Sporanox), voriconazole (Vfend), posaconazole (Noxafil), ketoconazole, amphotericin B, caspofungin (Cancidas), micafungin (Mycamine), and anidulafungin (Eraxis).
The bottom line
Saccharomyces boulardii is a yeast and systemic antifungals kill yeast. Stop the probiotic during antifungal therapy. Switch to a bacterial probiotic if you need one, or wait until the antifungal course is finished and your physician approves restarting.