Florastor is a probiotic used primarily to prevent and treat diarrhea, especially the kind caused by antibiotics. Unlike most probiotics on the market, Florastor contains a yeast rather than bacteria, which gives it a unique advantage: antibiotics can’t kill it. The active ingredient is a specific yeast strain called Saccharomyces boulardii CNCM I-745, manufactured by the French pharmaceutical company Biocodex.
Why It’s a Yeast, and Why That Matters
Most probiotics contain bacterial strains like Lactobacillus or Bifidobacterium. Florastor is different. Because Saccharomyces boulardii belongs to the fungal kingdom, it is naturally resistant to antibiotics. Antibiotics target bacteria, so they wipe out both harmful and helpful bacterial strains in your gut. A yeast-based probiotic survives that process entirely intact, continuing to work even while you’re on a course of antibiotics.
This is the main reason Florastor is so commonly recommended alongside antibiotic treatment. Bacterial probiotics taken at the same time as antibiotics may be partially or fully destroyed before they can do anything useful.
Preventing Antibiotic-Associated Diarrhea
The most well-studied use for Florastor is preventing diarrhea that develops as a side effect of antibiotics. Roughly one in three people taking antibiotics will experience loose stools or full diarrhea because the medication disrupts the balance of gut bacteria. Taking Florastor during your antibiotic course helps maintain that balance, reducing the likelihood and severity of digestive upset.
Many doctors suggest starting Florastor on the same day you begin antibiotics and continuing for several days after the course ends, since your gut flora needs time to recover. Because the yeast isn’t affected by antibiotics, you can take it at the same time as your antibiotic dose, though some physicians prefer patients space them a few hours apart out of general caution.
Protecting Against Traveler’s Diarrhea
Florastor is also used to help prevent traveler’s diarrhea, the digestive illness that strikes people visiting regions with unfamiliar food and water. A systematic review and meta-analysis published in the Journal of Infection and Public Health found that Saccharomyces boulardii was among the probiotic strains effective at reducing the risk of traveler’s diarrhea. People typically start taking it a few days before departure and continue throughout the trip.
How It Works in Your Gut
Florastor protects your digestive system through several overlapping mechanisms. One of the most important is its effect on your gut’s immune defenses. The yeast stimulates production of secretory IgA, the most abundant immune protein in your intestinal lining. Secretory IgA acts as a first line of defense: it prevents harmful microorganisms from attaching to the gut wall, neutralizes them, and pushes them out of your digestive tract. This is a non-inflammatory process, meaning it protects you without causing the swelling and irritation that other immune responses can trigger.
Research across multiple animal models, including mice and calves, has consistently shown that Saccharomyces boulardii supplementation boosts this IgA response and promotes the development of a more diverse, well-connected gut microbiome. The yeast also appears to block colonization by harmful bacteria like certain strains of E. coli, helping crowd out potential troublemakers before they can establish themselves.
Other Digestive Uses
Beyond antibiotic and travel-related diarrhea, Florastor is used for general intestinal recovery. People take it for acute diarrhea from infections, chronic diarrhea related to inflammatory bowel conditions, and to support gut health after illness. Some people use it during and after stomach bugs to speed up the return to normal digestion. It is also sometimes recommended for people dealing with recurring Clostridioides difficile (C. diff) infections, a serious and stubborn bacterial infection of the colon that often follows antibiotic use.
How to Take Florastor
The standard adult dose is one capsule or packet taken twice daily. You can swallow the capsule whole, or open it and mix the contents into about four ounces of water, juice, or milk. It also works sprinkled over soft foods like applesauce or yogurt, which makes it easier for children or anyone who has trouble swallowing capsules. Avoid mixing it with very hot food or drinks, since high temperatures can kill the live yeast.
For best results during antibiotic treatment, continue taking Florastor for at least a few days after your last antibiotic dose. Your gut bacteria don’t bounce back instantly, and the yeast provides a bridge of protection during that vulnerable recovery window.
Who Should Avoid Florastor
Florastor is generally well tolerated, but it carries real risks for certain people. The CDC has documented cases of Saccharomyces fungemia, a yeast bloodstream infection, in patients who are critically ill, receiving tube feeding, or have a central venous catheter (a line placed into a large vein, often in the chest or neck). In these situations, the yeast can potentially enter the bloodstream and cause a serious infection.
People with severely compromised immune systems should also avoid Florastor. If you’re hospitalized or have an indwelling catheter, the risk-benefit calculation is very different from that of someone at home dealing with a standard antibiotic prescription. Anyone with a known yeast allergy should skip it as well, since the active ingredient is a living fungal organism.
What to Expect When Taking It
Most people notice no side effects at all. Occasional mild bloating or gas is possible in the first day or two as your gut adjusts. Florastor doesn’t permanently colonize your intestines. The yeast passes through your system within about five days of your last dose, which means it works as a temporary resident that supports your native bacteria rather than replacing them. This transient nature is actually part of its safety profile: it does its job and then clears out on its own.