Supplements › Other / not yet classified

Probiotics

Live bacteria and yeasts. Results depend on the strain. Studied for diarrhea after antibiotics, irritable bowel syndrome, colds and more.

Strongest evidenceGut healthStrong, 587 studies
Median dose in studies6 billion CFU a dayFrom 877 study groups
CFU per dose in supplements1 to 10 billionSome contain 50 billion or more. More CFU is not necessarily more effective.
Upper limitNone listedNIH fact sheet. People with a weak immune system or a serious illness should ask first.
Ask a doctor or pharmacist first

Ask a doctor or pharmacist first if you:

  • have a weak immune system or a serious illness: rare blood infections from probiotic microbes have been reported, mostly in these people[1]
  • are in intensive care, are fed through a tube or have a central venous catheter: many reported cases of yeast in the blood had these risks[1]
  • are pregnant: a 2021 Cochrane review found more pre-eclampsia, a high blood pressure disorder of pregnancy, with probiotics[2]
  • plan to give a probiotic to a premature baby: the US FDA warns of invasive, possibly fatal infections from the microbes in these products[3]

Overview

What are probiotics?

Probiotics are live microorganisms that give a health benefit when taken in adequate amounts.[1][4] Most are bacteria, and some are yeasts. They are found in some fermented foods such as yogurt, are added to other foods and are sold as supplements.[1]

Probiotics are not one substance. Each one is a specific strain, named by genus, species and a strain code, as in Lacticaseibacillus rhamnosus GG.[1] If one kind of Lactobacillus helps prevent an illness, that does not necessarily mean that another kind will do the same.[5] In 2020, scientists split the old genus Lactobacillus into 25 genera, so some strains now have new names.[6]

Prebiotics are something else: they are carbohydrates that gut microbes use as fuel. Synbiotics contain both prebiotics and probiotics. Postbiotics are made of dead or broken microbes.[1]

What are probiotics used for?

Mostly for the gut. The results depend on the strain and on the health problem, so a result from a review of many probiotics does not apply to every product.[1][7] What reviews found:

  • Diarrhea from antibiotics. A 2025 Cochrane review found it in 23% of people who took probiotics and 27.4% of the others (low certainty).[8]
  • Irritable bowel syndrome (IBS). In 82 trials, some strains and mixes may help, but the certainty was low to very low for almost all results.[9]
  • Constipation. In 30 trials in adults, 57% improved with probiotics and 44% without. Stool frequency rose with Bifidobacterium lactis, but not with several other products.[10]
  • Colds. A review of 23 trials found that fewer people had upper airway infections (low certainty).[11] See immune support.
  • Mood. Reviews found less depression and anxiety, but the trials differ widely and their quality is limited.[12][13]
  • Sleep. A review of 15 trials found better scores for sleep quality, with some risk of bias.[14]
  • Weight. Results are inconsistent, and the average effects are small.[1]
  • Iron. In a pooled analysis of 8 studies, Lactobacillus plantarum 299v raised iron absorption. Other strains did not improve iron status.[15] See essential health.
  • Skin. A Cochrane review found little or no difference in symptoms of eczema.[16]
  • Gums. In 14 studies of periodontitis, a gum disease, adding probiotics to usual treatment lowered plaque and bleeding.[17]
  • Fatigue. Of 13 trials with 513 athletes and active people, 10 reported improvements in measures such as endurance or gut symptoms. The authors concluded that at least 15 billion colony-forming units (CFU, live cells) a day for at least 28 days may reduce fatigue.[18]
  • Allergic rhinitis (hay fever). A review of 28 studies found less severe symptoms, but the results varied widely.[19]

Are probiotics safe, and who should avoid them?

For most healthy people, probiotics are unlikely to cause harm, and side effects such as gas are usually minor.[1] Rare blood infections from the microbes have been reported, mostly in people who were severely ill or had a weak immune system.[1] Pregnant women and parents of premature babies should also ask a doctor first.[2][3]

How it works

Probiotics are live microbes taken by mouth, and they act in the digestive tract.[1] Different types may have different effects: a result for one kind of Lactobacillus does not necessarily carry over to another kind.[5]

  1. Gut passage

    In a study of an 11-strain mix, the microbes were still alive after they passed through the gut.[20]

  2. Short stay

    Probiotics may settle on the gut lining for a short time. How well they settle depends on the person, the strain and the part of the gut.[1] The effect of the 11-strain mix was short and differed from person to person.[20]

  3. Shared effects

    Many strains may slow the growth of harmful microbes, make short-chain fatty acids and make the colon more acidic.[1]

  4. Species effects

    Other effects belong to certain species. These include making vitamins, supporting the gut barrier, breaking down bile salts and neutralizing toxins.[1]

  5. Strain effects

    Effects on immune signals, hormones and the nervous system differ from strain to strain. For this reason, advice on use needs to name the strain.[1]

Benefits and studies

Each row is one benefit. The square shows the grade and the number of studies. Open a row to see the most trusted studies.

StrongModerateWeakNo trials
BenefitMain effects in studiesStudiesPeople

Dose

No single dose fits all probiotics: the best dose depends on the strain and the product.[1] Doses are counted in colony-forming units (CFU), the number of live cells. Many supplements contain 1 to 10 billion CFU per dose, and some contain 50 billion or more.[1] Expert bodies make no recommendation for or against probiotics in healthy people.[1]

Usual dose

It depends on the goal and the strain. To prevent diarrhea from antibiotics in children with risk factors, a European pediatric society recommends at least 5 billion CFU a day of Lactobacillus rhamnosus GG or Saccharomyces boulardii.[1] Most trials on colds gave one or two strains at 1 billion or 100 billion CFU a day.[11] For irritable bowel syndrome, the strain, dose and duration are not yet confirmed.[1]

Form

Probiotics are sold as capsules, powders, liquids and other forms, with one strain or many.[1] US labels must list the total weight of the microbes. This weight includes dead cells and says nothing about the live count. CFU on the label is voluntary. Probiotics can die during storage, so the NIH Office of Dietary Supplements points to labels that give the CFU at the end of shelf life.[1]

When to take

For diarrhea from antibiotics, starting within 2 days of the first antibiotic dose worked better than starting later.[1] Some products need a refrigerator and others do not, and the label gives the storage conditions.[1]

How long

One trial on diarrhea from antibiotics gave a probiotic until 7 days after the last antibiotic dose.[21] Most trials on colds lasted more than 3 months.[11] In a review of 10 trials on irritable bowel syndrome, treatment lasted 4 weeks to 6 months.[1]

Upper limit

The NIH fact sheet lists no upper limit for probiotics, and it says that products with more CFU are not necessarily more effective.[1] People with a weak immune system or a serious illness, pregnant women and parents of premature babies should ask a doctor or pharmacist first.[1][2][3]

Questions about the doseIs a higher CFU count better? ›

Doses in studies

Most studies gave 5 to 10 billion CFU a day, for 1 to 3 months.

The counts are study groups. A study group is one set of people in a study who took the same dose. One study can have several groups. These are amounts from studies, not advice for you.

How much a day877 groups in 701 studies
  1. Up to 0.05 billion CFU21 groups
  2. 0.05 to 0.1 billion CFU37 groups
  3. 0.1 to 0.25 billion CFU32 groups
  4. 0.25 to 0.5 billion CFU26 groups
  5. 0.5 to 1 billion CFU103 groups
  6. 1 to 2.5 billion CFU107 groups
  7. 2.5 to 5 billion CFU105 groups
  8. 5 to 10 billion CFU214 groups
  9. 10 to 25 billion CFU102 groups
  10. 25 to 50 billion CFU47 groups
  11. 50 to 100 billion CFU38 groups
  12. 100 to 250 billion CFU20 groups
  13. 250 billion CFU or more25 groups
For how long1,029 groups in 844 studies
  1. 1 week or less63 groups
  2. 1 to 4 weeks322 groups
  3. 1 to 3 months522 groups
  4. 3 to 6 months85 groups
  5. 6 to 12 months26 groups
  6. More than a year11 groups
Half of the groups took less than 6 billion CFU a day. Half took more.Half of the groups took 1.077 to 14.25 billion CFU a day. A quarter took less, a quarter took more.

Taken together

218 of 1105 study groups took probiotics with something else. This shows what studies gave together, not what works together.

  1. Prebiotics62 studies66
  2. Inulin19 studies20
  3. Vitamin D12 studies13
  4. Selenium10 studies12
  5. Zinc9 studies11
  6. Omega-3 Fatty Acids10 studies10
  7. Fiber8 studies8
  8. Calcium5 studies7
  9. Xylitol7 studies7
  10. Fish Oil6 studies6
Bars and numbers count study groups. A companion is shown only when at least 3 different studies gave it.

Protocols

What studies used when they gave probiotics for one goal, or to a group of people.

Each protocol describes what studies used, for one goal or for one group of people. It is not advice for you: talk to a doctor before you start a supplement.

Questions

12 questions in 4 groups. Open one for the short answer; each has its own page with the full answer and sources.

Basics
Which foods contain probiotics?

Some yogurts do, and some foods have probiotics added. Yogurt is made with Lactobacillus bulgaricus and Streptococcus thermophilus, and it may also contain probiotics from Bifidobacterium or the Lactobacillaceae family.[1] Milks, juices, smoothies, cereals, nutrition bars and infant formulas may have probiotics added, usually named on the label with a strain code and a CFU count.[1]

Not every fermented food counts:[1]

  • Sourdough bread and most commercial pickles are processed after fermentation and have no live cultures.[1]
  • Many cheeses, kimchi, kombucha, sauerkraut, miso and raw apple cider vinegar contain live cultures, but they do not usually contain proven probiotics.[1]
Open the answer with sources ›
Benefits
Do probiotics prevent diarrhea from antibiotics and C. difficile?

They may, but the effect depends on the strain, the age of the person and the risk. In a 2025 Cochrane review, diarrhea during antibiotics affected 23% of people on probiotics and 27.4% of the others. C. difficile diarrhea affected 1.6% and 3.2%. Both results had low certainty.[8] Saccharomyces boulardii and Lactobacillus rhamnosus GG have been shown to lower the risk.[1] A large trial in hospital patients aged 65 and older found no benefit from a mix of other strains.[22]

Read the full answer ›
Do probiotics help IBS?

Some strains may help a little, but the evidence is weak. A 2023 review of 82 trials with 10,332 adults with irritable bowel syndrome (IBS) found possible benefits for some strains and mixes. The certainty of the evidence was low to very low for almost all results.[9] No strain, dose or duration is confirmed yet.[1]

Read the full answer ›
Do probiotics help with bloating and gas?

Perhaps in people with irritable bowel syndrome (IBS), but the certainty is very low. In a 2023 review of 82 IBS trials, the evidence that mixes and Bacillus strains help bloating had very low certainty.[9] In 10 trials with 877 adults with IBS, gas fell with all the probiotics tested. Abdominal distension (a swollen belly) improved with products that contained Bifidobacterium breve, Bifidobacterium infantis, Lactobacillus casei or Lactobacillus plantarum.[1] In another comparison, 3 strains and 2 mixes reduced bloating.[23] These reviews studied people with IBS, so their results may not apply to other people.[9] Gas can also be a side effect of probiotics.[1]

Open the answer with sources ›
Do probiotics help with constipation?

Some may help adults, by a modest amount. A 2022 review of 30 trials in adults with long-term constipation found that 57% improved with probiotics and 44% without.[10] Stool frequency rose with Bifidobacterium lactis, but not with mixes of probiotics, Bacillus coagulans Unique IS2 or Lactobacillus casei Shirota. Stool consistency did not change, and products that also contained prebiotics did not help. The trials varied widely and many had a risk of bias.[10]

In children the evidence is unclear: two 2017 reviews of small trials reached different conclusions.[5]

Open the answer with sources ›
Do probiotics shorten diarrhea from an infection?

Large, well-run trials do not show it. A 2020 Cochrane review included 82 studies with 12,127 people, most of them children. In the trials with a low risk of bias, probiotics probably made little or no difference to diarrhea that lasted 48 hours or longer (risk ratio 1.00). The effect on its length was uncertain.[24] When all the trials were pooled, diarrhea that lasted 48 hours or longer was 36% less common and diarrhea was 21.3 hours shorter, but the results varied widely between trials.[1][24] No serious side effects were linked to probiotics.[24]

Expert groups disagree. A European pediatric society gives a weak recommendation for a few named strains, based on low or very low certainty evidence. The American Gastroenterological Association recommends against probiotics for children with this illness in the US and Canada.[1]

Open the answer with sources ›
Do probiotics prevent colds?

They may lower the risk. A 2022 Cochrane review of 23 trials with 6950 people studied acute upper airway infections, such as colds:[11]

  • People with at least one infection: 24% fewer (low certainty).[11]
  • People with at least three infections: 41% fewer (moderate certainty).[11]
  • Length of an infection: 1.22 days shorter (low certainty).[11]
  • People who needed antibiotics for these infections: 42% fewer (moderate certainty).[11]

Most trials used one or two strains, such as Lactobacillus plantarum HEAL9 or Lactobacillus paracasei 8700:2 or N1115, for more than 3 months. These results may not apply to other strains.[1] Side effects were minor and mostly affected the gut.[11] The trial details are in the protocol Probiotics for colds.

Open the answer with sources ›
Do probiotics help with depression and anxiety?

Reviews found less depression and anxiety, but the trials are mixed and many have limits. A 2025 review covered 23 trials with 1401 people with a diagnosis. With up to 8 weeks of use, it found a substantial drop in depression symptoms and a moderate drop in anxiety symptoms. The results differed widely between trials.[12] A larger 2025 review of 72 trials, which also included prebiotics, synbiotics and people with chronic disease or healthy people with symptoms, found smaller drops. Its authors said that the high variation and limited quality call for larger trials.[13]

The effect varied by group, duration and type of probiotic, so these results do not apply to every strain.[13] Both reviews describe probiotics as a possible addition to usual treatment.[12][13] See stress and mood.

Open the answer with sources ›
Do probiotics help you lose weight?

Only slightly, if at all, and the trials disagree.[1] Reviews of mixed strains found:

  • 15 trials with 957 people: 0.6 kg more weight lost than with placebo. The NIH fact sheet calls this effect small and of doubtful importance.[1]
  • 19 trials with 1412 people: a waist 0.82 cm smaller, and no change in weight or body mass index (BMI).[1]
  • 8 trials with 412 people with obesity: more weight lost and less belly fat, but no difference in BMI.[25]

Single strains also differ. In 210 adults with a lot of belly fat, fermented milk with Lactobacillus gasseri SBT2055 lowered belly fat by about 8% in 12 weeks.[1] In 125 adults with obesity, Lactobacillus rhamnosus CGMCC1.3724 for 24 weeks did not change weight loss overall, although women lost 2.6 kg more than with placebo.[1] See weight management.

Open the answer with sources ›
Safety
Can you take probiotics during pregnancy?

Ask a doctor first, because one review found a possible harm. A 2021 Cochrane review of 7 trials with 1647 women found more pre-eclampsia, a high blood pressure disorder of pregnancy, with probiotics (risk ratio 1.85, 4 studies with 955 women, high certainty).[2] It was uncertain whether probiotics lower the risk of diabetes in pregnancy, and the authors urged caution.[2]

An earlier review found no change in the risk of preterm birth (risk ratio 1.08, 11 studies with 2484 women).[26]

Open the answer with sources ›
Are probiotics safe for babies?

It depends on the baby. For premature babies there are serious warnings,[3] and in trials of other babies no serious side effects were reported.[27]

Premature babies. The US FDA warns that they can get invasive, possibly fatal infections from the microbes. It reported one death in 2023 and more than two dozen other adverse events since 2018. No probiotic has FDA approval as a drug for babies of any age.[3] The American Academy of Pediatrics does not support routine use, particularly under 1000 g at birth.[28] A Cochrane review of 60 trials found that probiotics may lower the risk of necrotizing enterocolitis, a serious bowel disease, in very premature babies (risk ratio 0.54, low certainty). In extremely premature babies, they may have little or no effect.[29] The World Health Organization says that probiotics may be considered for babies born before 32 weeks who get human milk.[1]

Other babies. In a Cochrane review of 24 studies with 7077 mothers and babies, there were no serious side effects, and the evidence was too limited to say that probiotics prevent allergies.[27] In babies with colic, Limosilactobacillus reuteri DSM 17938 cut crying and fussing by about 25 minutes a day at day 21, with a clear effect only in breastfed babies. This result applies to that strain.[30]

Open the answer with sources ›
Forms and timing
Does the probiotic strain matter, and how do you choose one?

Yes. A 2018 review of 228 trials found that the effect of a probiotic depends on the exact strain and on the health problem. A result for one product does not necessarily carry over to another.[7] Expert groups name strains for this reason, and they look for trials of that strain, at a known dose, for the same problem.[1][31]

Read the full answer ›

References

  1. Probiotics: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025. ods.od.nih.gov
  2. Probiotics for preventing gestational diabetes. Davidson SJ, Barrett HL, Price SA, Callaway LK, Dekker Nitert M Cochrane Database Syst Rev, 2021. PMID 33870484
  3. FDA Raises Concerns About Probiotic Products Sold for Use in Hospitalized Preterm Infants. US Food and Drug Administration, 2023. www.fda.gov
  4. Expert consensus document. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Hill C, Guarner F, Reid G, Gibson GR, Merenstein DJ, Pot B, et al. Nat Rev Gastroenterol Hepatol, 2014. PMID 24912386
  5. Probiotics: Usefulness and Safety. NIH National Center for Complementary and Integrative Health, 2019. www.nccih.nih.gov
  6. A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae. Zheng J, Wittouck S, Salvetti E, Franz CMAP, Harris HMB, Mattarelli P, et al. Int J Syst Evol Microbiol, 2020. PMID 32293557

Review history

  1. 4 Nov 2026CheckedReviewerChecked every claim against its source.
  2. 4 Nov 2026Page createdResearcherFirst edition, written from PubMed and public health sources.