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Lacticaseibacillus paracasei strain Shirota (formerly Lactobacillus casei strain Shirota)
The Yakult strain — the most-trialled single probiotic strain of the three psychobiotic-adjacent contenders, and a clear illustration of why trial size matters. The two largest trials (n=737 and n=359) were null on their primary endpoints. The positives cluster in smaller, Yakult-affiliated studies that compared fermented milk against plain milk.
What the evidence says
Most L. casei Shirota studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from medium-quality meta-analyses and randomised trials published 2003–2025 with a typical study size of 94 participants.
Based on 10 studies · 1 meta-analysis · 9 RCTs · 1,735 total participants
Confidence
High confidenceBy outcome
The largest evidence base of any strain in this group — and precisely why the score isn't higher. Both large trials were null on their primary endpoints and the one meta-analysis covering it is negative for this strain, while the positives cluster in smaller Yakult-affiliated trials that used unfermented milk rather than a matched placebo as the comparator.
No trials currently enrolling · 4 completed on ClinicalTrials.gov
Registered trials show research momentum for L. casei Shirota, not proof of effect — a registration is a plan, and posted results are sponsor-reported, not peer-reviewed. They are never counted toward the evidence rating above.
Browse these trials on ClinicalTrials.govClinicalTrials.gov · as of Sep 2026
Our research database did not respond in time, so the studies and count above cover only our curated set. This refreshes automatically.
Lacticaseibacillus paracasei strain Shirota — universally still called Lactobacillus casei Shirota, and sold worldwide as Yakult — has been studied for longer and in more people than almost any other single probiotic strain.
The 2020 taxonomic split of the genus Lactobacillus moved it from Lactobacillus casei to Lacticaseibacillus paracasei; the organism did not change, only the name.
Almost every trial delivered it as a fermented milk drink of 65 to 100 mL, not as a capsule, which matters twice over: capsule products do not automatically inherit this evidence, and the format itself is a confounder because several trials used plain unfermented milk rather than a matched placebo as the comparator.
The evidence divides sharply by trial size.
The two largest studies were both null on their primary endpoints: a 737-resident, 176-day nursing-home trial found no effect on respiratory symptoms (OR 0.87, 95% CI 0.62-1.29) and no effect on influenza vaccine response, and a 359-patient multicentre trial in spinal cord injury found no reduction in antibiotic-associated diarrhoea (RR 1.071, p=0.639) — its positive proton-pump-inhibitor result came from a pre-stated secondary analysis, not the primary endpoint.
A genuine meta-analysis of probiotics in chronic constipation found that Bifidobacterium lactis significantly increased stool frequency but that this strain specifically did not.
Against that, several smaller trials are positive: 96 male office workers had markedly fewer upper respiratory infections (22.4% vs 53.2%, P=0.002), 70 people with chronic constipation reported improvement (89% vs 56%, P=0.003), and 50 US adults with hard stools improved stool consistency (OR 0.34, P=0.014).
The most interesting positives are objective: overnight EEG in medical students under exam stress showed preserved N3 sleep and higher delta power, and gut microbiota alpha-diversity was preserved under the same stressor.
The pattern is consistent enough to justify a Moderate rating and clear enough to explain why it isn't higher — the effect, if real, is small enough that adequately powered independent trials cannot find it.
In medical students facing a national exam, 16S rRNA sequencing showed the fermented-milk group retained significantly higher species richness and a lower proportion of Bacteroidaceae than placebo. The effect is protecting diversity against a stressor rather than raising it at baseline.
In 200 office workers, HLA-DR expression on conventional dendritic cells and monocytes was significantly higher after 14 and 28 days, with higher CD86 on dendritic cells at day 14. A separate trial found the strain inhibited the stress-related fall in NK cell activity.
Salivary cortisol rose significantly before the exam in the placebo group only, in two separate stress trials. This is the most plausible route by which a gut-resident organism affects sleep architecture and abdominal symptoms.
How L. casei Shirota works — from molecular targets to health outcomes. Click an edge to see supporting research.This visualization is in beta — pathways are being refined and expanded.
Tested directly in 737 residents over 176 days with no benefit on respiratory symptoms or vaccine response. The working-age respiratory results do not extend here.
The studied format is a fermented milk drink. Non-dairy or capsule formats have no trial evidence for this strain.
The commercial drink contains added sugar in a small volume. Consider that alongside any digestive benefit.
Standard live-probiotic caution applies; no strain-specific data exists either way.
Not a harmful interaction — the opposite. In the 359-patient antibiotic-associated-diarrhoea trial, benefit appeared only in participants taking PPIs regularly (RR 0.54 at 30 days, p=0.015) while the overall primary endpoint was null. This was a secondary subgroup analysis — the trial listed the PPI question among its aims, so it is not post hoc, but it is not the primary endpoint either and needs prospective confirmation before it is acted on.
Tip: The 359-patient trial reported no intervention-related adverse events; the constipation trial reported no adverse reactions
Tip: The constipation trial found the occurrence and degree of flatulence and bloating did not change in either direction — this strain neither helps nor worsens them
L. casei Shirota has an evidence score of 5/10 — moderate evidence based on 15 indexed studies, including 1 meta-analysis. The Yakult strain — the most-trialled single probiotic strain of the three psychobiotic-adjacent contenders, and a clear illustration of why trial size matters. The two largest trials (n=737 and n=359) were null on their primary endpoints. The positives cluster in smaller, Yakult-affiliated studies that compared fermented milk against plain milk. Representative study: PMID 36372047.
The commonly studied dose of L. casei Shirota is One 65-100 mL bottle of the fermented milk drink daily, delivering roughly 6.5 x 10^9 to 1 x 10^11 CFU depending on the product. Doses across trials span this whole range with no dose-comparison study to separate them.. Individual needs vary — start at the lower end of the range and adjust based on how you respond.
Timing is flexible for L. casei Shirota — consistent daily use matters more than the time of day. No trial compared time of day or fed versus fasted dosing.
L. casei Shirota is generally well-tolerated and considered safe for most healthy adults at recommended doses. The most commonly reported side effects are bloating or flatulence unchanged, mild digestive upset. Use caution if any of these apply to you: Immunocompromise or immunosuppression (class-level precaution for live probiotics); Critical illness or indwelling central venous catheter.
Probiotics
Likely helpsLive microorganisms where strain selection determines outcome — Lactobacillus and Bifidobacterium strains best studied for gut, immune, and mood.
Saccharomyces Boulardii
Likely helpsAntibiotic-resistant probiotic yeast that supports digestive regularity and gut flora during travel and antibiotic use.
L. plantarum 299v
Mostly mechanism / observationalThe one probiotic strain with a genuinely distinctive trick: taken with a meal it increases non-heme iron absorption by roughly 50%, replicated across independent dual-isotope studies. Its IBS evidence is far weaker — rated low certainty, with one clearly negative trial — and it does not fix iron deficiency on its own.
VSL#3 / Visbiome
Mostly mechanism / observationalAn 8-strain blend dosed in the hundreds of billions to trillions of CFU per day — roughly 45 to 7,000 times a typical 1-10 billion CFU consumer probiotic — with the largest effect size in the probiotic literature for one narrow use: maintaining remission in chronic pouchitis after ulcerative colitis surgery. It is null for IBS and Crohn's — and since 2016 the brand name no longer reliably identifies the formulation that was studied.
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Reviewed by Dr. Baher Al Hakim · Last reviewed July 2026 · evidence from 10 studies · how we score · editorial policy
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.
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