S. boulardii, a living yeast probiotic, is used mainly against diarrhea. With antibiotics, it lowers diarrhea risk, though some large hospital studies found no protection, and modestly improves treatment of an ulcer-causing stomach bacterium. Its main harm, a rare but serious bloodstream yeast infection, concentrates in people with intravenous catheters, critical illness or weakened immunity. It looks most useful short-term alongside antibiotics, with little established benefit as a continuous supplement. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Complete blood count with absolute neutrophil count | 1.5–8.0 × 10⁹/L (standard reference range) | Safety check: low counts signal immunodeficiency, a contraindication |
| Blood cultures (fungal) | No established target; track whether any yeast grows | Safety check: positive culture means stopping use |
| Stool Clostridioides difficile toxin or PCR | No established target; track a positive result during new diarrhea | Recurrence is what the yeast aims to prevent |
| Helicobacter pylori urea breath test | No established target; track conversion from positive to negative after therapy | Confirms eradication |
Cadence: Baseline check before starting; daily symptom and temperature awareness during each course; stool review at 1–2 weeks; indication-specific retesting at the end of treatment (breath test at least 4 weeks after H. pylori therapy); immediate blood cultures for any unexplained fever; review every 3–6 months for long-term use