Does Florastor Cause Loose Stools?
No, Florastor (Saccharomyces boulardii) does not cause loose stools—it actually treats and prevents diarrhea across multiple clinical contexts. The evidence consistently demonstrates that S. boulardii reduces diarrhea duration, frequency, and severity rather than causing it.
Mechanism and Clinical Evidence
S. boulardii functions as an anti-diarrheal probiotic with robust evidence supporting its therapeutic role:
In acute gastroenteritis, S. boulardii reduces diarrhea duration by approximately 28.77 hours (95% CI: 40.35 fewer hours to 17.18 fewer hours) and decreases the risk of diarrhea lasting more than 4 days (RR 0.45; 95% CI 0.32-0.64) 1.
In children with acute diarrhea, treatment with S. boulardii significantly shortened recovery time from 95.3 hours in placebo to 65.8 hours (p=0.0001), with faster normalization of stool consistency 2. Another pediatric study demonstrated mean diarrhea duration of 4.70 days versus 6.16 days with placebo (p<0.05) 3.
For antibiotic-associated diarrhea prevention, S. boulardii reduced diarrhea prevalence from 23% to 8% (RR 0.3; 95% CI 0.2-0.7) in children receiving antibiotics 4.
In critically ill tube-fed patients, S. boulardii reduced the percentage of days with diarrhea from 18.9% to 14.2% (OR 0.67; 95% CI 0.50-0.90, p=0.0069) 5.
FDA-Labeled Indications
The FDA labeling for Florastor lists indications for "temporary relief of bowel congestions-constipation, gas, bloating" and "digestive disturbance," with no mention of loose stools as an adverse effect 6.
Clinical Practice Guidelines
The AGA Clinical Practice Guidelines (2020) recommend S. boulardii specifically for prevention of C. difficile infection in adults and children on antibiotic treatment, acknowledging its anti-diarrheal properties rather than any diarrhea-causing effects 7. The guidelines note that S. boulardii may reduce both cessation (RR 1.33; 95% CI 1.02-1.74) and recurrence of diarrhea (RR 0.59; 95% CI 0.35-0.98) in C. difficile infection, though evidence quality is low 7.
Recent Confirmatory Evidence
A 2025 randomized controlled trial demonstrated that S. boulardii combined with vancomycin for C. difficile infection achieved a 96.6% global cure rate versus 85.3% with vancomycin alone (p=0.044), with significantly lower recurrence (1.7% vs 13.1%, p=0.032) and no increase in adverse events 8.
Important Caveats
One negative study in elderly hospitalized patients found no benefit from S. boulardii for antibiotic-associated diarrhea, though this did not demonstrate harm or loose stools as a side effect 9.
The FDA warnings mention only standard precautions about persistent symptoms and keeping medication away from children, with no specific warnings about diarrhea or loose stools 6.
If a patient reports loose stools while taking Florastor, consider alternative causes such as the underlying condition being treated, concurrent medications (especially antibiotics), dietary factors, or other gastrointestinal pathology rather than attributing it to the S. boulardii itself, as the evidence overwhelmingly supports its anti-diarrheal rather than pro-diarrheal effects.