Most clinical trials showing a benefit from probiotics during antibiotic use have tested doses in the range of 10 to 20 billion colony-forming units (CFU) per day for adults. But the dose that works depends heavily on which strain you’re taking, because not all probiotics have the same evidence behind them. Here’s what the research actually supports.
Which Strains Have the Strongest Evidence
Two probiotics stand out in clinical trials for preventing antibiotic-associated diarrhea: Lactobacillus rhamnosus GG (often labeled LGG) and Saccharomyces boulardii, a beneficial yeast. Both are recommended by the European Society for Paediatric Gastroenterology and the American Gastroenterological Association, though with the caveat that evidence quality is moderate at best.
The AGA also recognizes a few multi-strain combinations: L. acidophilus CL1285 paired with L. casei LBC80R, and a three- or four-strain blend containing L. acidophilus, B. bifidum, and L. delbrueckii subspecies bulgaricus (with or without S. salivarius subspecies thermophilus). These combinations showed benefit specifically for preventing C. difficile infection, a more serious complication of antibiotic use. Other strains and blends lack sufficient clinical trial data to recommend confidently.
Recommended Doses for Adults
For Lactobacillus rhamnosus GG, the strongest results in trials came from doses of 10 to 20 billion CFU per day. In children, that same dose range reduced the risk of antibiotic-associated diarrhea by 71%, and adult trials have used comparable amounts.
For Saccharomyces boulardii, dosing is typically measured in milligrams rather than CFU. Clinical trials used daily doses up to 1,000 mg for adults. A practical target is 250 to 1,000 mg per day, which matches the range tested in randomized controlled trials. Most widely available S. boulardii supplements contain 250 mg per capsule, so two to four capsules daily falls within the studied range.
Doses for Children
European pediatric guidelines recommend the same two strains for children but at adjusted doses. For LGG, the best outcomes came at 10 to 20 billion CFU per day, the same as adults, though lower doses were also tested with weaker results. For S. boulardii, the recommended range for children is 250 to 500 mg per day. These guidelines note that the truly optimal pediatric dose hasn’t been established for either strain, so staying within the range used in trials is the safest approach.
When to Take Probiotics Around Your Antibiotic Dose
No study has directly compared different timing strategies, so there’s no proven “best” schedule. That said, the International Scientific Association for Probiotics and Prebiotics (ISAPP) suggests spacing your probiotic at least two hours away from your antibiotic dose. The logic is straightforward: most bacterial probiotics are sensitive to the same antibiotics you’re taking, so swallowing them together means the antibiotic may kill the probiotic before it can do anything useful.
There’s one notable exception. Saccharomyces boulardii is a yeast, not a bacterium, so antibiotics don’t affect it at all. If you’re using S. boulardii, you can take it at the same time as your antibiotic without reducing its effectiveness. This is one reason many clinicians favor it as a companion to antibiotic therapy.
How Long to Continue After Antibiotics
Start the probiotic on the same day you begin your antibiotic course. Most trials had participants continue the probiotic for the full duration of antibiotic treatment and then for several days to a couple of weeks afterward. The exact post-antibiotic duration hasn’t been studied head-to-head, but continuing for at least a week after your last antibiotic dose is a common practice based on how long gut disruption typically persists. Your gut microbiome can take weeks or even months to fully recover from a course of antibiotics, but most probiotic trials didn’t extend supplementation that long.
How Probiotics Protect Your Gut During Antibiotics
Antibiotics don’t just kill the bacteria causing your infection. They also wipe out large populations of beneficial gut bacteria, which can lead to diarrhea, bloating, and in some cases, overgrowth of harmful organisms like C. difficile. Probiotics work through several overlapping mechanisms to limit that damage.
They produce antimicrobial compounds that keep harmful bacteria from expanding into the space left behind by your normal gut flora. They also compete with pathogens for nutrients and for attachment sites along the intestinal lining, essentially occupying the real estate before dangerous microbes can move in. On a chemical level, probiotics generate organic acids like lactate and acetate that lower the pH in your gut, creating an environment that favors beneficial bacteria over pathogens. Those same acids serve as raw material for other gut bacteria to produce butyrate and propionate, short-chain fatty acids that nourish intestinal cells.
S. boulardii has an additional trick: it helps normalize bile acid composition. Antibiotics like amoxicillin-clavulanate shift bile acids toward a profile associated with gut irritation. In healthy volunteers, adding S. boulardii reversed those bile acid changes, restoring a more typical balance. Some probiotic strains also directly improve fluid absorption in the intestine, which is the most immediate way they reduce diarrhea symptoms.
Who Should Be Cautious
Probiotics are generally well tolerated in healthy adults and children, but they carry real risks for certain groups. People with compromised immune systems, critically ill patients, very young infants, and older adults with severe illness have developed bloodstream infections from probiotic organisms. These cases are rare, but they can be serious and even fatal. If you have a condition that weakens your immune system, or if you’re hospitalized with a severe illness, the potential harm from probiotics may outweigh the modest benefit of preventing diarrhea.
The AGA guidelines explicitly note that patients who are severely ill, or who place a high value on avoiding even small risks, may reasonably choose to skip probiotics altogether. For otherwise healthy people taking a standard outpatient antibiotic course, the risk-benefit calculation is more favorable, but the benefit itself is moderate rather than dramatic.
Choosing a Product
Probiotic supplements are not regulated with the same rigor as prescription drugs, so quality varies between brands. Look for a product that names the specific strain on the label (not just the species), lists the CFU count at the time of expiration rather than at manufacture, and comes from a manufacturer with third-party quality testing. A product labeled simply “Lactobacillus” or “probiotic blend” without strain-level detail gives you no way to match it against clinical evidence. The strains that performed well in trials are LGG (specifically Lactobacillus rhamnosus GG) and S. boulardii CNCM I-745. Generic versions of these organisms may or may not behave the same way.