Best Probiotic Strains: Ranked by Clinical Evidence

The best probiotic strains are the ones with human trial evidence for your specific condition — not the ones with the highest CFU count or the longest ingredient list. Across the strongest evidence base, five strains recur: Saccharomyces boulardii CNCM I-745, Lactobacillus rhamnosus GG, Bifidobacterium longum 35624, Lactiplantibacillus plantarum 299v and Bifidobacterium lactis BB-12.

That is the whole argument of this page in one paragraph. Probiotic benefit is demonstrated strain by strain and condition by condition, so evidence for one strain says nothing about another strain in the same species.^[1] A systematic review of 228 randomised controlled trials covering 25 probiotic types found exactly this pattern — in one comparison, a Lactobacillus casei strain cut antibiotic-associated diarrhoea to 6.7% while a different strain of the same species left it at 33.3%.

Below is a chart of 30 clinically studied strains ranked by the strength of evidence behind them, with the dose used in trials and — where the trial strain is actually purchasable — which product carries it. We have no affiliate relationships and receive nothing from any brand named on this page.

Medical disclaimer: This article is for information only and is not medical advice. Doses shown are the amounts used in published trials, not recommendations. Consult a qualified healthcare provider before starting a probiotic, particularly if you are immunocompromised, pregnant, critically ill, or taking medications.

Probiotic strain comparison chart: 30 clinically studied strains

How to read the evidence column

A grade describes how much evidence exists for that strain and that outcome — nothing else. A grade A strain for antibiotic-associated diarrhoea is not a grade A strain for anything else.

The chart

How to use this chart. Products are named to help you identify which retail item actually contains the strain that was tested — not as endorsements. Doses are the amounts used in the cited trials, not recommendations. Where a strain is supplied as an ingredient brand rather than a standalone product, the culture company is named instead, because the same strain appears under many different retail labels. Responses vary substantially between individuals depending on existing microbiome composition, diet and health status.^[14]

For a lighter, genus-by-genus quick reference, see our probiotic strain comparison chart. For help reading any bottle, see how to evaluate a microbiome supplement.

A note on names. Most Lactobacillus species were reclassified into new genera in 2020.^[15] L. rhamnosus is now Lacticaseibacillus rhamnosus, L. plantarum is now Lactiplantibacillus plantarum, and L. reuteri is now Limosilactobacillus reuteri. Product labels and older trials still use the traditional names, so both appear on this page.

Human strain vs. soil-based vs. dairy-derived probiotics

This is one of the most common questions in probiotic selection, and one of the least well served by the supplement industry — because these category labels do a great deal of marketing work while carrying almost no predictive information.

What "human strain" actually means on a label

"Human strain" or "human origin" means the organism was first isolated from a human body, usually the intestine, mouth or vagina, rather than from soil, plants or dairy. It is a descriptive claim about provenance. It is not a regulated term, there is no standard defining it, and no certification verifies it.

More to the point, origin is not one of the criteria scientific bodies use to qualify an organism as a probiotic at all. Those criteria are that the organism is sufficiently characterised, safe for its intended use, supported by at least one positive human clinical trial conducted to accepted standards, and alive at an effective dose through the end of shelf life.^[16] The consensus definition of a probiotic says nothing whatsoever about where the organism came from.^[17]

The clearest way to see the problem is by counterexample. Two of the best-evidenced probiotics in existence are not human in origin:

• S. boulardii CNCM I-745 — a yeast isolated in 1923 from the peel of lychee and mangosteen fruit in Indochina. It carries grade A evidence for antibiotic-associated diarrhoea.^[2]
• Live yoghurt cultures — S. thermophilus and L. bulgaricus are dairy fermentation organisms, and their effect on lactose digestion is one of the very few probiotic-related health claims European regulators have approved.

If human origin predicted efficacy, neither of those should work. Both do.

Does human origin predict colonisation?

This is the strongest version of the human-strain argument: an organism from a human gut should be better adapted to a human gut and therefore more likely to take up residence. It is a reasonable hypothesis. It has been tested, and it does not hold as a rule.

When researchers gave healthy volunteers a multi-strain supplement and then sampled the gut lining directly by endoscopy rather than relying on stool, they found colonisation was person-specific rather than product-specific. Some people were "permissive" and the organisms established along the gut; others were "resistant" and cleared them, despite receiving identical products. Baseline host and microbiome features predicted which group a person fell into. Critically, whether the strains showed up in stool did not reliably indicate whether they had colonised the mucosa — meaning the stool-based measures most studies and consumer tests rely on can mislead.^[18]

There is a second problem with treating colonisation as the goal: for most documented probiotic benefits, colonisation is not the mechanism. Most strains are transient, effects generally fade within weeks of stopping, and the benefit appears to come from what the organism does while passing through.^[14][19]

Soil-based organisms and Bacillus spore-formers

Soil-based organisms — usually marketed as SBOs — are almost always Bacillus species: B. coagulans, B. subtilis, B. clausii, B. licheniformis. Their central advantage is genuine, but it is mechanical rather than clinical: they are supplied as dormant spores, so they survive stomach acid without enteric coating and remain stable at room temperature without refrigeration.

That advantage has been directly documented. B. clausii spores have been shown to survive gastrointestinal transit and persist for days after dosing,^[11] and B. subtilis DE111 has been shown to germinate in the human small intestine in a placebo-controlled crossover study.^[12] Surviving transit is a real, verified property of this category.

Three caveats belong alongside it:

1. The evidence base is much smaller. Lactobacillus and Bifidobacterium strains have decades of trials and multiple Cochrane reviews behind them. Most Bacillus strains have a handful of trials each — and surviving transit is a prerequisite for a benefit, not proof of one.
2. Genus-level safety is not uniform. Bacillus cereus is a well-known cause of food poisoning and sits in the same genus as the probiotic species, which makes accurate species identification more consequential here than elsewhere.
3. Label accuracy in this category has been a documented problem. Independent sequencing studies of commercial probiotics have repeatedly found products containing organisms not on the label, or missing organisms that were.^[20][21]

None of that makes SBOs a bad category. It makes them a category where the strain designation and the manufacturer matter more, not less.

Dairy-derived strains

Dairy fermentation organisms — L. bulgaricus, S. thermophilus, many Lactococcus species, and a good number of Lactobacillus strains isolated from cheese and fermented milk — are historically the best-characterised probiotics, because the dairy industry has been selecting and sequencing them for a century.

The practical considerations are about the carrier, not the organism. A dairy-carried probiotic is unsuitable for someone with a milk protein allergy. It is generally not a problem for lactose intolerance, since the cultures themselves consume lactose. Fermented dairy also delivers these organisms within a food matrix that may buffer stomach acid, though the amount actually delivered in food products often falls short of the doses used in trials. See probiotic foods for what a food-first approach can and cannot deliver.

The practical takeaway

Origin is a marketing axis. Strain-level trial evidence is the decision axis. Ask: does this exact strain designation have published human trials for the outcome I care about, at the dose printed on the label? That question is answerable from the chart above. "Human strain" is not.

For the same reasoning applied to the organisms that get the most press but are hardest to actually buy, see our guide to next-generation probiotics and what is genuinely available.

Which probiotic strains are actually clinically studied?

"Clinically studied" is the most abused phrase in probiotic marketing, because it is technically true of almost any organism while telling you nothing. Here is what we require before treating a strain as clinically studied on this site.

1. Strain-level specificity. Benefits demonstrated for L. rhamnosus GG cannot be assumed for other L. rhamnosus strains. We cite evidence at the strain designation, never at the species level. This is not pedantry — it is the finding the largest systematic review of the field was built on.^[1]

2. Human trials, not mechanism. Animal and in vitro work explains how something might work. It does not establish that it does. We weight strains with published randomised controlled trials or systematic reviews in humans.^[22]

3. Replication across research groups. A single positive trial is a starting point. Strains with repeated positive results from independent groups carry more weight than strains with three trials from the company that sells them.

4. The trialled dose. A strain studied at 10 billion CFU is not evidence for a product containing 1 billion CFU of the same strain. Dose-response in probiotics is inconsistent and strain-dependent — higher is not reliably better, but different is genuinely different.^[23]

What we deliberately ignore: brand reputation, celebrity endorsement, the number of strains in a product, CFU count in isolation, and affiliate compensation — of which we have none.

One honest complication. Even expert bodies reading the same trials reach different conclusions, because they weight certainty of evidence differently. Clinical guidelines on probiotics from different societies disagree with one another more than most readers realise, so knowing that a guideline exists matters less than knowing what it actually concluded and how confident it was.^[24]

Does a probiotic with more strains work better?

Short answer: not on current evidence, and the strain count is one of the least informative numbers on the bottle.

The most direct test came from a 2021 systematic review that compared single-strain trials against trials of mixtures containing the same matched strain, within the same disease indication. It covered 65 randomised controlled trials — 41 single-strain, 22 mixtures, 2 with both arms — across eight indications in 10,863 participants. Multi-strain mixtures were not significantly more effective than single strains.^[25]

It is worth being honest that this is not fully settled. Earlier reviews found some evidence favouring mixtures in particular settings,^[26] and others have argued that head-to-head comparisons are frequently confounded, since mixtures often deliver a higher total dose.^[27] What no one argues is that adding strains reliably adds benefit.

Three practical consequences:

• "10-strain" tells you a count, not a test. It says ten organisms are in the capsule. It does not say that combination has ever been studied together, or that any of them are present at a trialled dose.
• Proprietary blends hide the number that matters. If a label gives one total CFU across ten strains, you cannot tell whether the strain you want is 90% of the dose or 1% of it. Per-strain CFU is the disclosure to look for.
• Some mixtures genuinely are the evidence. The eight-strain De Simone formulation for pouchitis and the Bio-K+ combination for C. difficile prevention in hospital settings were tested as formulations, and both appear in guideline recommendations.^[10] In those cases the mixture is the studied unit — but that evidence belongs to that exact formulation, not to multi-strain products as a class.

Best probiotic strains by condition

Antibiotic-associated diarrhoea and post-antibiotic recovery

This is the strongest indication in the field. S. boulardii CNCM I-745 and L. rhamnosus GG both have meta-analysis support.^[3][5] S. boulardii is a yeast and therefore unaffected by antibacterial antibiotics, so it can be taken concurrently; bacterial strains are usually spaced from the antibiotic dose.

The two are not equally consistent, though. When trials are stratified by strain rather than pooled by genus, LGG did not reach significance for preventing antibiotic-associated diarrhoea in adults across six pooled trials, while S. boulardii did.^[1] LGG's strongest evidence is in children, particularly for acute gastroenteritis.^[4]

One nuance is routinely omitted from probiotic guides and deserves stating plainly. Preventing diarrhoea during a course of antibiotics and restoring a microbiome after one are different goals with different evidence. In a study that sampled the gut lining directly, people who took a probiotic after antibiotics showed delayed and persistently incomplete return of their own native gut bacteria compared with people who recovered unaided.^[28] That finding comes from one research group and one protocol, so it should not be over-read — but it is a real reason not to assume a probiotic is automatically the right post-antibiotic move.

See our gut restoration protocol and our guide to probiotics for diarrhoea.

Clostridioides difficile

For prevention of C. difficile-associated diarrhoea in people taking antibiotics, a 2017 Cochrane review of 31 trials in 8,672 participants found incidence fell from 4.0% in control groups to 1.5% with probiotics (RR 0.40, 95% CI 0.30–0.52), at moderate certainty of evidence.^[2] The absolute benefit is concentrated in people at high baseline risk; in low-risk populations there is very little to prevent.

The AGA's 2020 guideline suggests S. boulardii, or specific two-, three- and four-strain Lactobacillus and Bifidobacterium combinations, for prevention in adults and children on antibiotics — as a conditional recommendation based on low certainty of evidence.^[10]

No probiotic is a treatment for an established C. difficile infection. If you have or suspect one, that is a medical issue rather than a supplement question. See C. difficile infection and Clostridioides difficile.

Irritable bowel syndrome

B. longum 35624 has the most consistent single-strain record, anchored by a 362-person randomised trial in which 1×10⁸ CFU (100 million) daily outperformed both placebo and the higher and lower doses of the same strain — a useful reminder that more is not reliably better.^[6] L. plantarum 299v has repeated trials for pain and bloating, and meta-analysis supports probiotics as a class in IBS while noting substantial heterogeneity between trials.^[7]

A correction to what is widely repeated online: the AGA's 2020 guideline did not recommend any probiotic for IBS. It concluded there was insufficient evidence to make a recommendation and suggested probiotic use in IBS be confined to the context of a clinical trial.^[10] Strain-level trial evidence for 35624 and 299v exists and is worth knowing about; a guideline endorsement does not.

See IBS, the IBS protocol, best probiotic strains for IBS and probiotics for bloating.

E. coli, Klebsiella and other opportunists

Searches for "probiotics for E. coli" or "probiotics for Klebsiella" usually conflate two different things, and the honest answer differs for each.

Most E. coli in a healthy gut is a harmless commensal, and one E. coli strain — Nissle 1917 — is itself a probiotic with trial evidence for maintaining remission in ulcerative colitis. Pathogenic E. coli infection is an entirely different matter, and no probiotic has evidence as a treatment for it.

For Klebsiella pneumoniae, gut overgrowth has been associated with several inflammatory conditions in observational research, but there is currently no probiotic strain with human trial evidence for reducing Klebsiella abundance. Products marketed for this purpose are extrapolating from in vitro inhibition data. We would rather say that plainly than imply an answer exists.

The same caution applies to a test result flagging a single "bad" organism — see our guide to reading a microbiome test result and our overview of opportunistic bacteria.

Microscopic colitis

Probiotic trials in microscopic colitis are few, small, and have not produced consistent benefit; budesonide remains the established therapy. This is a condition where the appropriate answer is that current evidence does not support a probiotic recommendation, and where diagnosis and management belong with a gastroenterologist. Related reading: inflammatory bowel disease and chronic diarrhoea.

Immune support and respiratory infections

A Cochrane review found probiotics were associated with fewer people experiencing acute upper respiratory tract infection episodes and with shorter illness duration, though it rated the quality of the evidence low to moderate.^[29] L. rhamnosus GG and B. lactis BB-12 are the most-studied strains here.

The effect is real but modest, and probiotics are not a substitute for vaccination, sleep or a decent diet. See common cold and the respiratory and immune support protocol.

Vaginal and urinary health

The L. rhamnosus GR-1 + L. reuteri RC-14 pair has the most evidence among oral probiotics for vaginal outcomes, with trials showing increased vaginal Lactobacillus colonisation after oral dosing — the strains transit from the GI tract.^[9] Evidence supports use as an adjunct in bacterial vaginosis rather than as a standalone treatment, and evidence for recurrent UTI prevention is weaker than commonly claimed.

See bacterial vaginosis, urinary tract infections, the women's vaginal and urinary protocol and probiotics for BV and yeast infections.

Constipation and regularity

Meta-analysis of trials in adults with functional constipation found probiotics modestly improved whole-gut transit time and stool frequency, with B. lactis strains accounting for much of the effect.^[8] Effect sizes are small. See constipation and probiotics for constipation.

Metabolic health

Akkermansia muciniphila is the most discussed next-generation candidate. A proof-of-concept trial in overweight and obese volunteers found pasteurised A. muciniphila was well tolerated and associated with improvements in some metabolic markers.^[13] This is a single exploratory study with a small sample — grade D on our scale — and should be read as promising rather than established. See Akkermansia muciniphila and the metabolic health protocol.

Finding a product that contains the strain you want

Identifying the right strain is only half the problem. Buying it is the other half, and it is harder than it should be, because most trial strains are ingredient brands licensed to many different retail labels.

Which products contain L. rhamnosus GG?

LGG is an ingredient brand, currently supplied by Chr. Hansen/Novonesis. In the US the best-known single-strain LGG product is Culturelle; in Europe it appears in Dicoflor and in various Valio dairy products, and it is licensed into a number of multi-strain supplements.

The label check that matters: it must print "Lactobacillus rhamnosus GG" or "LGG". A label reading only "Lactobacillus rhamnosus" with no strain code is, as far as the evidence is concerned, a different organism — even though it is the same species.

Which product contains both LGG and S. boulardii?

There is no widely available single product that combines both trial strains — L. rhamnosus GG and S. boulardii CNCM I-745. They come from different suppliers, and one is a bacterium while the other is a yeast, which complicates a shared formulation and shelf-life specification.

Products listing "Lactobacillus rhamnosus" and "Saccharomyces boulardii" without strain codes are common, and they are not the same thing. If you want both trial strains, they are generally taken as two separate products. If the goal is antibiotic cover specifically, S. boulardii is the one that can be taken at the same time as an antibacterial antibiotic, because antibacterials do not act on yeast.

How to verify a strain on a label

1. Look for genus, species and strain designation. "Lacticaseibacillus rhamnosus GG" is complete. "Lactobacillus blend" is not.
2. Look for per-strain CFU, not just a total. A proprietary blend reporting one combined figure across eight strains is withholding the number you need.
3. Look for CFU guaranteed through expiry, not at manufacture. Viability declines over shelf life, sometimes steeply.
4. Check storage requirements — and whether the product was actually stored that way in the shop.
5. Prefer third-party verified products (USP, NSF, ConsumerLab) where available. Independent sequencing studies have repeatedly found mismatches between what probiotic labels claim and what capsules contain.^[20][21]

Red flags: claims to cure or treat a disease; proprietary blends that hide per-strain amounts; very high CFU counts presented as the main selling point; and vague "supports overall wellness" language with no strain-specific research cited.

Once you have chosen a product, the next question is how long to give it. Timelines vary by goal — days for acute diarrhoea, but 4 to 8 weeks before immune or mood effects can be fairly judged. See our guide to how long it takes for probiotics to work for the goal-by-goal breakdown, and signs probiotics are working for what to watch for. For a full walkthrough of a label line by line, see how to evaluate a microbiome supplement; for dosing specifically, see the CFU guide.

Safety: who should not self-prescribe

Probiotics are well tolerated by most healthy adults, but "generally safe" is not "safe for everyone." People who are severely immunocompromised, critically ill, have central venous catheters, or have short bowel syndrome should avoid probiotics or use them only under medical supervision. Genomic work has traced probiotic organisms from capsules into the bloodstream of intensive care patients — a small but real risk in that population.^[30]

Mild gas and bloating are common in the first few days and usually settle. If symptoms worsen or persist, stop and speak to a clinician.

The bottom line

The best probiotic strain is not a product — it is a strain designation with human trial evidence for the outcome you care about, taken at the dose those trials used. That reframing does more work than any ranking. Use the chart above to find the strain, then use the label checks to confirm a product actually contains it.

Two related pages worth reading next: prebiotics vs. probiotics for whether you need the fibre side of the equation, and the microbiome database for detail on individual organisms.