L. reuteri
Also sold as: Reuteri, Lactobacillus reuteri, Limosilactobacillus reuteri
A specific probiotic species with genuinely strain-specific effects. Worth separating from general probiotics because in this field the strain is the drug — two products both saying 'L. reuteri' can behave completely differently.
L. reuteri quick facts
| Suggested dose | Typically 1–10 billion CFU daily. Match the strain to the outcome you want — DSM 17938 and ATCC PTA 6475 are studied for different things. |
| How often | Daily |
| Who it's for | Post-antibiotic recovery, H. pylori protocols, infant colic. |
Match the strain to the goal — a product labeled only 'L. reuteri' with no strain designation tells you nothing usable. 1–10 billion CFU daily, and probiotics generally need continued use because colonization is transient in most adults. The home-fermented 'reuteri yoghurt' protocol extrapolates mouse hormone data to humans and adds a real contamination risk at the long fermentation times used. Immunocompromised people should not take live probiotics without medical advice — bacteremia is rare but documented.
How L. reuteri actually works
Produces reuterin, an antimicrobial that suppresses gram-negative organisms including H. pylori, and adheres to intestinal mucus to compete for binding sites. The strain determines the effect entirely: DSM 17938 is the colic strain, ATCC PTA 6475 the immunomodulatory one. Rodent work suggests vagal signaling to oxytocin release, which is the basis of the current online interest and has no human confirmation.
Where to get L. reuteri
Find L. reuteri on iHerb →The evidence for L. reuteri
Graded by what exists behind each claim.
✅ Clinically validated
- DSM 17938 has good randomized evidence for reducing crying time in breastfed infants with colic — one of the better-supported probiotic indications there is.
- Used as an adjunct in Helicobacter pylori eradication, several trials report improved tolerability and modestly improved eradication rates.
- Trials in adults for general wellbeing outcomes are far weaker than the marketing suggests.
📊 Correlative data
- L. reuteri appears to be less prevalent in modern Western guts than in historical or non-industrialized populations, which is the observation the current interest is built on.
🧪 Theoretical / extrapolated benefits
- Rodent work links L. reuteri to oxytocin signaling via the vagus, with downstream effects reported on social behavior, wound healing and testicular size. This is the basis of the 'reuteri yoghurt' trend and it has NOT been demonstrated in humans.
How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →
What L. reuteri actually does
The organism has been renamed, and the new name is on none of the bottles. Lactobacillus reuteri was reclassified into the genus Limosilactobacillus, and the current clinical literature uses that name Dargenio 2022. A product still labeled Lactobacillus reuteri is not necessarily wrong — it is using a superseded name, which is a small signal about how closely the manufacturer follows the field.
The species' signature chemistry is a two-substrate reaction, and the second substrate is not in the capsule. L. reuteri carries a vitamin B12-dependent glycerol dehydratase that converts glycerol to 3-hydroxypropionaldehyde, the compound known as reuterin, which is broadly antimicrobial. No glycerol, no reuterin. That single dependency explains why in vitro results with this species can be spectacular and why a swallowed capsule, arriving in a gut whose free glycerol is low, is a different situation entirely.
Strain-level genetics decide which functions a given product has. Reuterin production, bile salt hydrolase activity, exopolysaccharide production and immunomodulatory signaling are carried on gene clusters that differ between strains of the same species. This is why the clinical reviews are written about a numbered deposit — DSM 17938 — rather than about L. reuteri Dargenio 2022. The species name is a family; the deposit number is the individual.
The vagal-oxytocin story that drives current interest comes from mice, and the mechanism the authors proposed is anti-inflammatory rather than hormonal. Male mice given purified lactic acid bacteria in drinking water had larger testes and higher serum testosterone, with increased seminiferous tubule cross-sectional profiles, spermatogenesis and Leydig cell numbers — and the same restoration of youthful testicular features was achieved with systemic antibody blockade of interleukin-17A Poutahidis 2014. The authors' own reading is that uncontrolled host inflammation drives the aged phenotype and that the microbe reduces it.
That distinction changes what the finding predicts. If the pathway is inflammation, then the effect should depend on how inflamed the host is, should be reproducible by other anti-inflammatory means, and should be largest in an aged, inflamed animal — which is what the mouse work found Poutahidis 2014. It does not predict a testosterone rise in a healthy young man, and nobody has looked.
Cell, rodent, human — and where it stops
The strongest single data point in this species' literature is negative, and it is the one the marketing never mentions. A double-blind, placebo-controlled randomized trial gave 1 × 108 colony forming units a day of DSM 17938 or placebo for one month to 167 breastfed and formula-fed infants under three months meeting Wessel's criteria for crying or fussing, recruited from a community sample rather than a specialist clinic Sung 2014.
The probiotic group cried or fussed MORE. At one month the probiotic group cried or fussed 49 minutes a day more than placebo, 95% confidence interval 8 to 90 minutes, P = 0.02, mainly reflecting fussing and especially in formula-fed infants; all secondary outcomes were similar, adherence was high, and 127 of 167 (76%) were retained to the primary outcome Sung 2014. The authors state that these findings differ from previous smaller trials of selected populations and do not support a general recommendation.
The meta-analytic picture is therefore split by population, not by belief. The randomized evidence for DSM 17938 in preterm newborns and infants with colic has been pooled systematically Bianchi 2026, and the pediatric review of this strain across gastrointestinal disorders is written strain-by-indication for the same reason Dargenio 2022. Breastfed infants in specialist settings and formula-fed infants in the community are not one population.
Where the same strain has a clean, large, positive result, the indication is a different one. In a prospective, multicenter, randomized, double-blind, placebo-controlled trial, 654 outpatient children on amoxicillin-clavulanate received DSM 17938 at 2 × 108 CFU or placebo. Antibiotic-associated diarrhea occurred in 7.9% versus 16.7% at 14 days (relative risk 0.47, 95% CI 0.30 to 0.7, P < 0.001), 8.8% versus 17.9% at 21 days and 9.1% versus 19.6% at 56 days, with the effect concentrated in children aged 6 to 24 months Dinleyici 2025.
So the translation rule for this species is unusually strict: the strain travels, the indication does not. DSM 17938 nearly halved antibiotic-associated diarrhea in children Dinleyici 2025 and did not reduce crying in a community colic sample Sung 2014. Both are the same organism at a similar dose. Anyone quoting one of those results as evidence for the other is doing the thing this page exists to name.
And the human obstacle to the testosterone claim is total. The testicular finding is in aging mice, on drinking water, with an inflammatory mechanism Poutahidis 2014. There is no human trial, no dose, no duration and no measured endpoint. The distance between that paper and a person fermenting yogurt at home is not a translation gap; it is an unexplored continent.
L. reuteri — which form, and does it matter
The deposit number is the product. Everything else on the label is packaging. DSM 17938 and ATCC PTA 6475 are two different deposits of Limosilactobacillus reuteri with two different evidence bases, and the clinical literature is organized around the numbered strain rather than the species Dargenio 2022. A product naming only “L. reuteri” has told you the genus and species and withheld the only identifier that carries evidence.
Colony forming units at manufacture and at end of shelf life are different numbers. The position paper on commercial probiotic products was written because declared counts, strain identity and viability were repeatedly not what labels claimed, and it calls explicitly for improved quality control Kolacek 2017. A count guaranteed through the expiry date, on a specific strain, refrigerated or not as the manufacturer specifies, is the minimum honest label.
The doses in the good trials are smaller than the shelf suggests. The colic trial used 1 × 108 CFU a day Sung 2014 and the antibiotic-associated diarrhea trial used 2 × 108 CFU a day Dinleyici 2025. Those are 0.1 and 0.2 billion. Products advertising 10 to 50 billion CFU are not delivering a hundred times the studied effect; they are outside the range in which anything was measured.
Drops, chewables and capsules are formulation choices with real consequences. An oil suspension protects the organism from moisture and is the format used in infant work; a chewable exposes it to moisture and heat; a capsule with an enteric strategy is a different delivery again. Viability is a property of the finished product and its storage, which is exactly what the quality-control position paper is about Kolacek 2017.
The home-fermented version is a different product, and the ways it differs are not favorable. A long high-temperature ferment selects for whatever grows fastest, not for the strain that was inoculated; the final count is unmeasured; and no organism identity check is possible in a kitchen. The whole argument of the quality position paper — that strain identity and viable count have to be verified for a probiotic claim to mean anything Kolacek 2017 — applies with more force, not less, to a preparation nobody tested.
What would have to be true, and how you would know it was not
1. The prediction that the popular version of this product fails. Predict no change in total testosterone, free testosterone, LH or SHBG in an adult man taking any L. reuteri preparation for 12 weeks, because the testicular result is a mouse result with an interleukin-17A mechanism and no human replication Poutahidis 2014. Markers: total testosterone, LH, SHBG. Retest window: 12 weeks, sampled before 10 a.m. on two separate days. This is the cheapest possible test of the claim and almost nobody making it has run it.
2. Predict the inflammatory marker moves before the hormone does, if either does. If the mouse mechanism transfers, the pathway is inflammation Poutahidis 2014, so predict hs-CRP would fall before testosterone rose. A study showing a testosterone rise with no change in any inflammatory marker would contradict the proposed mechanism rather than support it.
3. Predict the strain-specific indication behaves as published. In a child starting amoxicillin-clavulanate, predict roughly 8 cases of antibiotic-associated diarrhea per 100 on DSM 17938 against roughly 17 per 100 without it over 14 days Dinleyici 2025. Predict no such benefit from a different strain, and none from a product that does not name its deposit Dargenio 2022.
4. The prediction that cuts against the product in its most popular use. In a formula-fed infant with colic, predict no reduction in daily crying and, on the community trial's own numbers, the possibility of more fussing Sung 2014. Marker: a validated cry-fuss diary over 7 days. Retest window: one month. A parent who measures rather than remembers is running the trial's own primary outcome at home.
5. Predict most of the first two weeks is gas, and predict it settles. A colonizing organism ferments; predict bloating and wind for one to two weeks and predict it resolves without dose escalation. Persisting symptoms past three weeks are information, not a phase to push through.
What nobody has tested yet
Nobody has run the human study behind the trend. A randomized, placebo-controlled trial of a defined L. reuteri strain in adult men with morning total testosterone and hs-CRP as co-primary endpoints does not exist. The mouse paper is twelve years old Poutahidis 2014, the protocol built on it is everywhere, and the trial has never been attempted.
Nobody knows whether glycerol co-administration changes anything. Reuterin production requires glycerol, and that is a testable intervention: same strain, with and without a glycerol vehicle, with fecal reuterin or an antimicrobial readout as the endpoint. The biochemistry has been known for decades and the human experiment has not been done.
Nobody has explained the direction of the colic result. The community trial found more fussing on the probiotic, concentrated in formula-fed infants Sung 2014, while pooled analyses of selected populations look different Bianchi 2026. Whether feeding type genuinely modifies the effect, and why, is unresolved and is the most clinically useful open question in the strain's literature.
And nobody has audited the home-ferment. Somebody could sequence thirty batches of home-fermented reuteri yogurt and report what actually grew. Given the quality-control problems documented in commercially manufactured probiotics Kolacek 2017, the result for an unmonitored kitchen ferment would be worth knowing before the next person makes one.
L. reuteri — its own safety story, not its category's
The one contraindication that is not theoretical is a live organism in someone whose barriers are down. Any viable bacterial product carries a bloodstream-infection risk in profound immunosuppression, critical illness, short bowel, or with a central venous catheter in place. That is a property of live-biotherapeutic products as a class rather than of this species in particular, and it is the reason probiotic use in those settings is a clinical decision and not a consumer one.
In healthy people the adverse effects are fermentative and self-limiting. Gas, bloating and looser stools in the first one to two weeks are the expected picture, and the large pediatric trial reported the strain as well tolerated across 654 children Dinleyici 2025. The community colic trial recorded no study-related adverse events Sung 2014.
The finding that deserves more attention than it gets is that a probiotic can make a symptom worse. In that trial the probiotic arm fussed 49 minutes a day more than placebo Sung 2014. Whatever the explanation, it is a reminder that adding a fermenting organism to a gut is an intervention with a direction, and the direction is not guaranteed.
Strain-dependent histamine handling is real and is the reason “probiotics” is not a safety category. Some lactic acid bacteria decarboxylate histidine to histamine and some degrade it, and which of those a product does is a strain-level property Dargenio 2022. Somebody with histamine intolerance who reacts badly to one L. reuteri product has learned about that product, not about the species.
The home ferment is the risk this page would most like to talk a reader out of. A long warm ferment with an unverified starter has no count, no identity check and no contamination control, and the reason commercial products are held to a quality-control standard at all Kolacek 2017 is that those three things determine whether a probiotic is a probiotic. A jar that smells fine is not a certificate.
Sources read for this page
- Sung V. Treating infant colic with the probiotic Lactobacillus reuteri: double blind, placebo controlled randomised trial. BMJ 2014 · PMID 24690625
- Bianchi M. Efficacy of Lactobacillus reuteri DSM 17938 in preterm newborns and infants with colic: a systematic review and meta-analysis of randomized controlled trials. Nutrition 2026 · PMID 42485904
- Dargenio VN. Use of Limosilactobacillus reuteri DSM 17938 in paediatric gastrointestinal disorders: an updated review. Benef Microbes 2022 · PMID 35212258
- Dinleyici EC. Effect of Limosilactobacillus reuteri DSM17938 to prevent antibiotic-associated diarrhea in children: prospective, multi-center, randomized, placebo-controlled clinical trial (PEARL Study). Eur J Pediatr 2025 · PMID 40488914
- Poutahidis T. Probiotic Microbes Sustain Youthful Serum Testosterone Levels and Testicular Size in Aging Mice. PLoS One 2014 · PMID 24392159
- Kolacek S. Commercial Probiotic Products: A Call for Improved Quality Control. A Position Paper by the ESPGHAN Working Group for Probiotics and Prebiotics. J Pediatr Gastroenterol Nutr 2017 · PMID 28644359
How you would know if it worked
There is no blood test for this one. That is not a criticism — it is a fact about the effect, and it changes how you should judge it.
- What to watch: Only the outcome your specific strain was studied for. Read the label first: for DSM 17938 in an infant that is crying minutes per day, and for the Helicobacter pylori adjunct it is tolerance of the regimen — nausea and stool form during treatment — rather than eradication, which is settled by a breath or stool test and not by how you feel.
- How long before it means anything: Two to four weeks for a colic read; the length of the eradication course for the other. A bottle that prints no strain designation gets no window, because nothing identifiable is being tested.
- What will fool you: The oxytocin, wound-healing and testicular findings driving this product's reputation are rodent work that has not been shown in humans, so anything noticed in mood or libido is self-scoring against a story. Infant crying also peaks at around six weeks and falls away on its own afterward.
Run it one variable at a time. Starting three things in one week means a result you cannot attribute, which is the same as no result.
L. reuteri — safety & side effects
- Gas and bloating early on. Some strains produce histamine-lowering effects; others do the opposite, so strain identity matters.
- Home-fermented high-count yoghurts are popular and are not a controlled product — the counts and the contamination risk are both unknown.
The same on every page it applies to. Read it here; it is not repeated research.
- Bloodstream infection has occurred in immunocompromised patients, in critical illness, and in people with central venous catheters. That is the one contraindication here that is not theoretical. Otherwise: expect one to two weeks of gas and bloating while things settle.
Not medical advice. If you take prescription medication or have a diagnosed condition, check this against it with a pharmacist or doctor — pharmacists are underused and free.
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Baseline first, then again at 8–12 weeks.
| Marker | What it’s watching for |
|---|---|
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| Ferritin | Iron is the first thing malabsorption takes |
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L. reuteri — frequently asked questions
What is L. reuteri?
A specific probiotic species with genuinely strain-specific effects. Worth separating from general probiotics because in this field the strain is the drug — two products both saying 'L. reuteri' can behave completely differently.
What is the suggested dose of L. reuteri?
Typically 1–10 billion CFU daily. Match the strain to the outcome you want — DSM 17938 and ATCC PTA 6475 are studied for different things. This is a general reference for education only — statements have not been evaluated by the FDA and this is not medical advice.
What are the researched benefits of L. reuteri?
DSM 17938 has good randomized evidence for reducing crying time in breastfed infants with colic — one of the better-supported probiotic indications there is.
Who is L. reuteri for?
Post-antibiotic recovery, H. pylori protocols, infant colic.
Where can I buy L. reuteri?
Coach Cam sources L. reuteri from vetted, top-rated brands on iHerb — use the buy link on this page.
What L. reuteri is used for
L. reuteri appears under 1 goal in the goal router.
Related Gut & Digestion supplements
Where this goes next
The pages here are the frameworks. The protocols — the dosing, the order to correct things in, the week-by-week schedule and what to retest — are inside Skool.
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