Probiotics work for a few jobs, so match the strain or skip the bottle

Key takeaways

• A 2019 Cochrane review of 33 trials found that probiotics given with antibiotics cut children’s diarrhea from 19% to 8%, with the best results at 5 billion or more live microbes a day⁴.
• For people at high risk of C. difficile, a 2017 Cochrane review found that taking probiotics with antibiotics cut C. diff diarrhea from about 12% to under 4%⁵⁶.
• The American Gastroenterological Association’s 2020 guideline backed probiotics for only three narrow uses and said four others, including IBS, belong in research studies for now⁷.
• At Boston Children’s Hospital, 6 of 522 intensive care patients given a probiotic developed bloodstream infections with the same strain that was in the capsule¹⁰.
• Prebiotic powders deliver a measured dose of compounds also found in onions and garlic; Europe’s food safety agency found that 12 grams a day of chicory inulin makes bowel movements more frequent²¹⁴.

Between 2009 and 2014, 522 patients in the intensive care units at Boston Children’s Hospital were given capsules of a famous probiotic, Lactobacillus rhamnosus GG¹⁰. Six of them developed a bloodstream infection caused by Lactobacillus¹⁰.

When researchers read the genes of the bacteria from those patients’ blood, they matched the bacteria in the capsules almost letter for letter¹⁰¹¹. Among more than 21,000 ICU patients who didn’t get the probiotic, only two had that kind of infection, and neither involved this species¹⁰.

Now the twist. A Cochrane review named that same species as one of the two best choices for keeping children on antibiotics from getting diarrhea⁴.

Both findings hold up. A probiotic isn’t a vitamin. It’s a living organism with a particular job, and whether it helps or harms depends on who takes it, why, and which strain is in the bottle.

This guide sorts the uses with real evidence from the rest, names the people who should ask before taking one, and looks at prebiotic supplements, the powders sold to feed your gut microbes.

A probiotic needs a name, a dose and a proven benefit

In 2014, an expert panel convened by the International Scientific Association for Probiotics and Prebiotics (ISAPP) agreed on what the word means. Probiotics are “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host”¹.

That sentence sets three tests. The microbes must be alive. There must be enough of them. And a study must show they help.

A probiotic’s full name has three parts: genus, species and strain³. In Lactobacillus rhamnosus GG, the “GG” is the strain. Strains within one species can behave differently, so ISAPP advises matching the exact strain to the benefit you want².

The panel did allow one shortcut. A product with enough of a well-studied species, such as certain Lactobacillus or Bifidobacterium, can say it “contains probiotics”¹. Any more specific promise needs evidence for that strain¹.

Labels list microbes in colony-forming units (CFUs), a lab count of live cells. The NIH Office of Dietary Supplements (ODS) says a higher count doesn’t necessarily mean a bigger benefit³. What matters is getting at least the dose that was tested, and ISAPP says that dose should be guaranteed through the “best by” date, not just at the factory².

Not every fermented food qualifies, either. Our article on fermented foods explains why.

Antibiotics are where probiotics have earned their place

Antibiotics kill helpful gut bacteria along with harmful ones³. That’s the opening a probiotic can fill.

Children. In 2019, a Cochrane team led by researcher Q. Guo pooled 33 trials with 6,352 children taking antibiotics⁴. Diarrhea struck 19% of children in the comparison groups and 8% of those given probiotics⁴. Treating about nine children prevented one case⁴.

The effect was stronger at high doses. In trials using 5 billion CFUs a day or more, 8% of children on probiotics got diarrhea, compared with 23% in the comparison groups⁴.

The reviewers singled out L. rhamnosus and the yeast Saccharomyces boulardii, at 5 billion to 40 billion CFUs a day⁴. None of the 24 trials that tracked side effects found serious harms from probiotics⁴. The reviewers added a caution: outside these trials, serious side effects have been reported in children who were severely ill or immune-weakened⁴.

Adults. The picture is weaker. ODS says these two probiotics might lower the risk of antibiotic diarrhea in people under 65, but not in older people³. Starting within two days of the first antibiotic dose seems to matter³.

C. difficile. Clostridioides difficile, or C. diff, is a bacterium that can cause severe diarrhea after antibiotics. A 2017 Cochrane review led by Joshua Goldenberg pooled 31 trials with 8,672 people⁵. With probiotics, 16 in 1,000 got C. diff diarrhea, compared with 40 in 1,000 without them⁵. Most of the people studied were adults in the hospital⁶.

That works out to about 42 people treated to prevent one case⁶. For patients whose starting risk topped 5%, cases fell from 11.6% to 3.5%, and about 12 needed treatment to spare one⁶. At lower risk, the benefit was unclear⁵.

The caveats are real. About two-thirds of the trials had a high or unclear risk of bias, meaning flaws in how they were run or reported⁶.

The AGA’s 2020 guideline gave a conditional yes for preventing C. diff with a few specific products, including S. boulardii and a two-strain mix of L. acidophilus CL1285 and L. casei LBC80R⁷.

The AGA panel graded that evidence low quality, a step below Cochrane’s “moderate”⁵⁷. It also said low-risk outpatients who care more about cost could reasonably skip probiotics, and so could seriously ill patients most worried about harms⁷.

One strain may help some colicky babies

Colic means long bouts of crying in a healthy young baby with no clear cause. It wears families down.

In 2018, researchers pooled the raw data from four trials of Lactobacillus reuteri DSM 17938 in 345 infants⁹. By day 21, babies on the probiotic cried and fussed about 25 minutes less per day than babies on a placebo, against a starting average of about 260 minutes⁹.

The benefit showed up in breastfed babies, who were two to three times as likely to improve⁹. Only one small trial included formula-fed babies, so no one knows if it helps them⁹. TheNNT, an evidence-summary website, rated the benefit as unclear, noting how few babies were studied⁹.

If you’re considering it, talk with your pediatrician first. Babies born early or with health problems face the infection risks described below²¹².

The gastroenterologists’ 2020 verdict was mostly “not yet”

The AGA guideline made eight recommendations⁷:

• A conditional yes for preventing C. diff in people on antibiotics, for preventing a dangerous bowel disease called necrotizing enterocolitis in premature babies, and for pouchitis, an inflammation of the pouch surgeons build after removing the colon.
• A conditional no for children in North America with stomach bugs. Our coming article on diarrhea covers why.
• Use only in research studies for treating an active C. diff infection, Crohn’s disease, ulcerative colitis and IBS.

“Not yet” isn’t the same as “proven useless.” The AGA called those four areas knowledge gaps⁷. ISAPP, a scientific group focused on probiotics, pointed out that the AGA’s evidence report left out 22 IBS studies, mostly because usable data couldn’t be pulled from them⁸. The AGA also didn’t review antibiotic diarrhea at all⁸.

For IBS, ODS says probiotics might ease symptoms, but results vary by strain, how long people take them and which symptom is targeted³. Major guidelines disagree about IBS, and our article on gut-healing claims walks through that split. Probiotics for mood are covered in our gut-brain article.

Match the strain to the goal

This table is our own summary. Ratings use our four-level evidence scale.

Goal	Strains studied	Evidence rating	Who should skip it or ask first
Prevent diarrhea from antibiotics in children	L. rhamnosus GG or S. boulardii, 5 to 40 billion CFUs a day4	Strong evidence4	Babies born early; seriously ill or immune-weakened children23
Prevent diarrhea from antibiotics in adults	L. rhamnosus GG, S. boulardii3	Promising but early: may help under 65, not older adults3	Seriously ill people; weakened immunity3
Prevent C. diff in high-risk hospital patients on antibiotics	S. boulardii; L. acidophilus CL1285 with L. casei LBC80R; some 3- and 4-strain mixes7	Strong evidence per Cochrane, though the AGA graded it low quality; benefit unclear at low risk567	ICU patients, people with central IV lines, weakened immunity71011
Treat an active C. diff infection	None recommended7	Not enough evidence7	Everyone: this needs medical treatment
Colic in breastfed babies	L. reuteri DSM 179389	Promising but early9	Babies born early or with health problems13
Pick a strain that eases IBS	Many tested; none stands out37	Not enough evidence to name one78	Anyone with warning signs should see a clinician first
Stomach bugs in North American children	Various7	Evidence says no, or unsafe7	Not recommended
Pouchitis after colon surgery	An 8-strain mix7	Promising but early7	Use under specialist care
Everyday wellness in healthy adults	None specified	Not enough evidence: no official recommendation3	Not needed

Live microbes can cause infections in fragile bodies

ODS says probiotics in healthy people may cause gas but rarely cause infections³. The trouble is concentrated in a few groups.

Critically ill patients. In the Boston study published in Nature Medicine in 2019, 1.1% of ICU patients given the probiotic developed Lactobacillus bloodstream infections, compared with 0.009% of those who didn’t get it¹⁰. One patient’s bacteria even evolved resistance to the antibiotic rifampin inside the body¹⁰.

All but one of the six had a central line, an IV tube threaded into a large vein¹¹. Since 2016, the hospital hasn’t allowed that probiotic for ICU patients with central lines¹¹. The NIH helped fund the work¹¹.

Premature babies. In the fall of 2023, the FDA warned hospitals after a baby weighing under 2.2 pounds died¹³. The infant had been given Evivo with MCT Oil, a probiotic used for preterm babies in hospitals, and developed sepsis, a life-threatening infection, from bacteria that genetically matched the product¹³.

By late October, the FDA had sent warning letters to Evivo’s maker, Infinant Health, and to Abbott over a hospital product called Similac Probiotic Tri-Blend¹². Evivo with MCT Oil was recalled, and Abbott agreed to stop selling its product¹².

The agency counted more than two dozen other U.S. reports of harm since 2018¹². It has not approved any probiotic as a drug for infants of any age¹².

The American Academy of Pediatrics doesn’t support routine probiotics for preterm babies, especially those under 1,000 grams, citing the lack of pharmaceutical-grade products in the U.S.¹³. That’s a real tension with the AGA’s 2020 nod⁷, and it’s a decision for the hospital team, never for home use.

Prebiotic powders: a measured dose of what onions already carry

Prebiotics are compounds that particular gut microbes use in ways that benefit you². Common examples include inulin, fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS) and chicory fiber².

The same compounds come from food. ISAPP lists onions, garlic and Jerusalem artichokes as sources of inulin and FOS, and cooked-then-cooled potatoes as a source of resistant starch². Our prebiotic foods article covers which foods hold the most.

So what does a powder add? A known dose. ISAPP notes that Western diets run low in prebiotics, so matching the amounts used in research may take a supplement or an enriched food².

The clearest benefit is regularity. In 2015, the European Food Safety Authority (EFSA) reviewed six studies with 86 people and concluded that at least 12 grams a day of chicory inulin increases how often you have a bowel movement¹⁴. Six small studies make that promising but early on our scale.

Labels don’t always tell the whole story. In 2022, ConsumerLab, a private testing company that also runs a certification program for brands, tested five prebiotic products¹⁵. The actual prebiotic fiber ranged from 26% to 96% of the amount of the listed prebiotic ingredient¹⁵. Servings ran from about 1 to 15 grams, at 12 to 85 cents per gram of prebiotic fiber¹⁵.

By our math, a 12-gram daily dose at those prices would cost about $1.44 to $10.20 a day.

Expect gas or bloating if you start with too much, ISAPP warns, so begin small and build up². If you have IBS, read our prebiotic foods article first; it explains why these same compounds can set off symptoms. Prebiotics paired with probiotics in one product, called synbiotics, get their own article in this series.

Three questions before you buy

Our guide to questions before paying for any supplement covers the basics. For probiotics, ask these three:

1. What’s the full strain name, and was that strain tested for my goal? “Probiotic blend” isn’t enough. Look for the genus, species and strain code, such as DSM 17938²³. Then check it against the table above.
2. Does the label promise the tested dose through the best-by date? A count “at time of manufacture” may not survive months on a shelf². Follow storage directions, since some products need refrigeration³.
3. Does anything about my health tip the balance toward harm? Run through the box above. If any line fits, talk with your clinician first.

Quality is a fair worry. The AGA noted that probiotics aren’t regulated as drugs in the U.S. or Europe and that trials often left out manufacturing details⁷.

Britain’s research agency notes they aren’t subject to mandatory testing there⁶. A coming article in this series covers how to judge probiotic products.

Respecting your microbes means picking the right tool

Natural health starts from a sound instinct: your gut is a living ecosystem, and what you feed it matters. The strongest probiotic research honors that instinct by being specific.

A targeted strain during a course of antibiotics fits the natural-health ideal of using the least needed, at the right moment, for a clear reason. A daily capsule “for gut health” with no stated goal doesn’t, especially when ODS notes that no official recommendation exists for healthy people³.

The same skepticism you bring to a drug company belongs here. Ask who tested the strain, in whom, and whether it matched your situation.

Questions readers ask

The bottom line

Probiotics have a real but narrow record. Specific strains help prevent diarrhea in children on antibiotics and C. diff in high-risk patients, and one strain may soothe colic in breastfed babies. For IBS, inflammatory bowel disease and everyday wellness, the evidence is thin or unsettled.

Match the strain and dose to a clear goal, or skip the bottle. If you’re critically ill, have a central line, have a weakened immune system, or are caring for a premature baby, talk with a clinician first. Prebiotic powders mostly offer a measured dose of what onions, garlic and cooled potatoes already provide.

Related reading

• Leaky gut can be measured, but most “heal your gut” protocols haven’t been tested
• A prebiotic soda has 2 grams of fiber, and half an onion may have more
• Six servings of fermented food a day reshaped gut microbes in a Stanford trial, so start with one
• Three questions to ask before you pay for a supplement
• Stool became FDA-approved medicine for one infection, and other uses are still experiments

Sources

1. Sanders ME. “The Definition of Probiotics Twelve Years Later” (on Hill C, Guarner F, Reid G, et al., “The International Scientific Association for Probiotics and Prebiotics Consensus Statement on the Scope and Appropriate Use of the Term Probiotic,” Nature Reviews Gastroenterology & Hepatology, 2014). Gut Microbiota for Health, September 17, 2014. gutmicrobiotaforhealth.com
2. International Scientific Association for Probiotics and Prebiotics. “Biotics” consumer and educational materials (probiotics, prebiotics, synbiotics). ISAPP, 2019–2024.
3. NIH Office of Dietary Supplements. “Probiotics: Fact Sheet for Consumers.” Updated November 3, 2023. ods.od.nih.gov
4. Guo Q, Goldenberg JZ, Humphrey C, El Dib R, Johnston BC. “Probiotics for the Prevention of Pediatric Antibiotic-Associated Diarrhea.” Cochrane Database of Systematic Reviews, April 30, 2019 (Cochrane summary). cochrane.org
5. Goldenberg JZ, Yap C, Lytvyn L, et al. “Probiotics for the Prevention of Clostridium difficile-Associated Diarrhea in Adults and Children.” Cochrane Database of Systematic Reviews, December 19, 2017. cochranelibrary.com
6. NIHR Evidence. “Probiotics Can Prevent Bacterial Diarrhoea in Hospital Patients Receiving Antibiotics.” National Institute for Health and Care Research, May 2018. evidence.nihr.ac.uk
7. Su GL, Ko CW, Bercik P, et al. “AGA Clinical Practice Guidelines on the Role of Probiotics in the Management of Gastrointestinal Disorders.” Gastroenterology, 2020;159:697–705. spg.pt
8. International Scientific Association for Probiotics and Prebiotics. “ISAPP Take-Home Points From American Gastroenterological Association Guidelines on Probiotic Use for Gastrointestinal Disorders.” June 15, 2020. isappscience.org
9. TheNNT. “Lactobacillus reuteri for Treatment of Infant Colic” (on Sung V, D’Amico F, et al., Pediatrics, 2018). March 2020. thennt.com
10. Yelin I, Flett KB, Merakou C, et al. “Genomic and Epidemiological Evidence of Bacterial Transmission From Probiotic Capsule to Blood in ICU Patients.” Nature Medicine, 2019;25 (Technion author copy). pubmed.ncbi.nlm.nih.gov
11. Boston Children’s Hospital. “Beware Probiotics in ICU Patients.” Answers, November 7, 2019. answers.childrenshospital.org
12. U.S. Food and Drug Administration. “Warning Letters Issued to Two Companies for Illegally Selling Probiotic Products to Treat Diseases in Preterm Infants.” FDA bulletin, October 26, 2023. content.govdelivery.com
13. HealthDay. “Following One Death, FDA Warns Hospitals About Giving Probiotics to Preemies.” October 3, 2023. healthday.com
14. EFSA Panel on Dietetic Products, Nutrition and Allergies. “Scientific Opinion on the Substantiation of a Health Claim Related to ‘Native Chicory Inulin’ and Maintenance of Normal Defecation by Increasing Stool Frequency.” EFSA Journal, 2015;13(1):3951 (DOAJ record). doaj.org
15. ConsumerLab.com. “Prebiotic Supplements” test results news release. October 28, 2022. consumerlab.com