Probiotics Guide (2026): Stop Buying Based on CFU Count
What a probiotic does depends on its exact strain, not the CFU count on the front. The best evidence is for preventing diarrhea from antibiotics, where probiotics cut the risk by about 40% across 63 trials. Our pick for adults: Florastor (S. boulardii), $1.93 a day at 2 capsules at the brand’s price.
By Erin Rose Updated October 19, 2026 How we score
Brand prices, checked October 19, 2026.
What to buy
For diarrhea from antibiotics, adults should take S. boulardii, a probiotic yeast. Florastor holds S. boulardii CNCM I-745, 250 mg a capsule.
Florastor
S. boulardii CNCM I-745 · $1.93 a day at 2 capsules
Pick by goal
- Travel: Florastor again. See best probiotics for travel.
- Vaginal health: Jarrow Fem-Dophilus. See best probiotics for vaginal health.
- IBS or bloating: Align. See best probiotics for bloating.
- Colds and infections: see best probiotics for immune support.
- Comparing brands: Seed vs Culturelle, Align vs Digestive Advantage, and cheaper Seed alternatives.
- On a testing list: see third-party tested probiotics.
- Ranked across goals: see the best probiotic.
What probiotics do
A review of 228 trials found that results belong to the exact strain and the problem it was tested for (McFarland 2018).
| Goal | Evidence | What trials found |
|---|---|---|
| Diarrhea from antibiotics | Moderate | Across 63 trials, about 40% fewer cases (Hempel 2012). In children, 8% got it with a probiotic against 19% without (Guo 2019). |
| Traveler’s diarrhea | Some | S. boulardii CNCM I-745 cut cases by about a fifth (McFarland 2019). |
| Infections in hospital | Small | LGG lowered gut and respiratory infections in 742 children in hospital (Hojsak 2010). |
| IBS | Mixed | Some strains seemed to help symptoms and pain, but which ones work is unclear (Ford 2018). |
| Vaginal flora | Small, mixed | GR-1 and RC-14 helped in early trials; newer ones found less. See vaginal health. |
Why the CFU count does not compare products
CFU counts live cells, and a bigger number is not always better. In the best-known IBS trial, 362 women took B. infantis 35624 at three doses: 100 million CFU a day beat a placebo, while 1 million and 10 billion did not (Whorwell 2006). For diarrhea from antibiotics, 5 billion or more a day worked better in children (Guo 2019). The dosage guide lists the trial doses.
Taking probiotics with antibiotics
In adults, S. boulardii cut diarrhea from antibiotics from 17% to 8% across 15 trials (Szajewska 2015, S. boulardii). LGG, the strain in Culturelle, cut it in children; in adults its effect was too uncertain to count on (Szajewska 2015, LGG). Start it within 2 days of the first antibiotic dose and take it through the course; the benefit was seen in adults up to 64, not in those 65 and older (NIH). For a child, or for a product on a testing list, choose Culturelle.
Culturelle Digestive Daily
LGG · $0.60 a day · USP Verified
How long do probiotics take to work?
It depends on the goal:
- Antibiotics: during the course.
- IBS: 4 weeks in the best-known trial; stop if it does not help.
- Vaginal flora: 28 to 60 days.
- Travel: every day of the trip.
Best time to take probiotics
With a meal, or up to 30 minutes before one. In a lab model of the stomach, probiotic bacteria survived best that way, with a meal that had a little fat, such as oatmeal with milk. Taken 30 minutes after a meal, far fewer survived (Tompkins 2011).
Are probiotics safe?
For most people, yes. In trials that reported them, side effects were no more common than on a placebo, though many trials reported harms poorly (Hempel 2011). Check these:
- A weak immune system or a serious illness: ask your doctor first.
- Pregnancy: S. boulardii has not been tested in pregnancy (Dugoua 2009). Ask your doctor first.
Questions people ask
Do probiotics need to be refrigerated?
Only if the label says so. Culturelle, Florastor and Align do not; Jarrow Fem-Dophilus 5B does. Keep any bottle out of heat.
What is the difference between prebiotics, probiotics, and postbiotics?
Probiotics are live microbes. Prebiotics are fibers, such as inulin, that feed the microbes already in your gut. Postbiotics are dead microbes, or their parts, that still have a health effect (Salminen 2021).
Sources
- McFarland LV, et al. “Strain-Specificity and Disease-Specificity of Probiotic Efficacy: A Systematic Review and Meta-Analysis.” Front Med (Lausanne). 2018. PubMed 29868585
- Hempel S, et al. “Probiotics for the prevention and treatment of antibiotic-associated diarrhea: a systematic review and meta-analysis.” JAMA. 2012. PubMed 22570464
- Guo Q, et al. “Probiotics for the prevention of pediatric antibiotic-associated diarrhea.” Cochrane Database Syst Rev. 2019. PubMed 31039287
- Szajewska H, Kołodziej M. “Systematic review with meta-analysis: Lactobacillus rhamnosus GG in the prevention of antibiotic-associated diarrhoea in children and adults.” Aliment Pharmacol Ther. 2015. PubMed 26365389
- Szajewska H, Kołodziej M. “Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea.” Aliment Pharmacol Ther. 2015. PubMed 26216624
- McFarland LV, Goh S. “Are probiotics and prebiotics effective in the prevention of travellers’ diarrhea: A systematic review and meta-analysis.” Travel Med Infect Dis. 2019. PubMed 30278238
- Hojsak I, et al. “Lactobacillus GG in the prevention of nosocomial gastrointestinal and respiratory tract infections.” Pediatrics. 2010. PubMed 20403940
- Ford AC, et al. “Systematic review with meta-analysis: the efficacy of prebiotics, probiotics, synbiotics and antibiotics in irritable bowel syndrome.” Aliment Pharmacol Ther. 2018. PubMed 30294792
- Whorwell PJ, et al. “Efficacy of an encapsulated probiotic Bifidobacterium infantis 35624 in women with irritable bowel syndrome.” Am J Gastroenterol. 2006. PubMed 16863564
- Tompkins TA, et al. “The impact of meals on a probiotic during transit through a model of the human upper gastrointestinal tract.” Benef Microbes. 2011. PubMed 22146689
- Hempel S, et al. “Safety of probiotics used to reduce risk and prevent or treat disease.” Evid Rep Technol Assess. 2011. PubMed 23126627
- Dugoua JJ, et al. “Probiotic safety in pregnancy: a systematic review and meta-analysis of randomized controlled trials of Lactobacillus, Bifidobacterium, and Saccharomyces spp.” J Obstet Gynaecol Can. 2009. PubMed 19646321
- NIH Office of Dietary Supplements. “Probiotics: Fact Sheet for Health Professionals.” ods.od.nih.gov
- Salminen S, et al. “The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics.” Nat Rev Gastroenterol Hepatol. 2021. PubMed 33948025