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Bifidobacterium longum subsp. longum 35624 (marketed as B. infantis 35624)
The probiotic strain behind Align, and the one most often called 'the evidence-based one' for IBS. The reality is thinner: four single-strain trials, two of them null, sharing overlapping authorship — and the large positive dose-ranging trial enrolled women only. The dose-response is genuinely inconsistent rather than a clean 'less is more'.
What the evidence says
Most B. infantis 35624 studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from high-quality meta-analyses and randomised trials published 2005–2026 with a typical study size of 118 participants.
Based on 10 studies · 2 meta-analyses · 5 RCTs · 995 total participants
Confidence
High confidenceBy outcome
One clearly positive, adequately powered IBS trial and a well-documented human mechanism — but the entire single-strain base is four trials totalling ~858 participants with overlapping authorship, two of them null, and the two independent meta-analyses disagree: the 2017 pooled analysis found no single-strain effect, while the 2026 strain-specific review lists 35624 among strains showing efficacy but publishes no symptom-by-symptom breakdown.
No trials currently enrolling · 3 completed on ClinicalTrials.gov
Registered trials show research momentum for B. infantis 35624, not proof of effect — a registration is a plan, and posted results are sponsor-reported, not peer-reviewed.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.
Bifidobacterium longum subsp. longum 35624 — still sold as 'B. infantis 35624' under the Align and Alflorex brands — is among the most-studied single probiotic strains for irritable bowel syndrome, and it has a genuine claim to being better evidenced than most.
A large multicentre dose-ranging RCT (n=362) found significant improvement in abdominal pain and global symptoms — though it enrolled women only, so the headline effect has not been demonstrated in men.
An influential 2009 independent review singled the strain out as the only probiotic with efficacy demonstrated in two appropriately designed studies. But the foundation is narrower than the marketing implies.
The entire single-strain human efficacy base is four trials totalling roughly 858 participants, two of which found no symptom benefit, and the same small group of investigators appears across several of them.
A 2017 meta-analysis found no effect for the single strain, attributing the pooled benefit to multi-strain products containing it.
A 2026 strain-specific systematic review with meta-analysis of randomised placebo-controlled trials does list this strain among those demonstrating efficacy in improving key IBS symptoms, but it publishes no symptom-by-symptom breakdown.
The dose story is genuinely unsettled: 100 million CFU beat placebo and every other arm in the dose-ranging trial, yet 10 billion CFU beat placebo in the earlier O'Mahony trial while failing in the dose-ranging trial — where that arm had acknowledged formulation problems — and 1 billion CFU was null in a 302-person trial.
Two trials also show the strain does not colonize or remodel the microbiome — it passes through, so dosing must be continuous.
IBS patients showed an abnormal inflammatory-skewed IL-10/IL-12 ratio at baseline. Feeding this strain normalized it, while a comparator Lactobacillus strain in the same trial did not — the basis of the strain-specificity claim.
Human volunteers showed increased IL-10 secretion and enhanced Foxp3 expression in peripheral blood. Dendritic cells secreted IL-10 but not IL-12p70 — the cellular machinery behind the cytokine shift.
Reduced plasma CRP across ulcerative colitis, chronic fatigue and psoriasis cohorts, showing the effect reaches the systemic and not just the mucosal immune system. Biomarker endpoints only.
The strain is recoverable in stool during dosing but returns to baseline once stopped, with only limited changes to other gut taxa. The effect is immune signalling during passage, which rules out the common 'rebuilds your gut flora' claim and means dosing must be continuous.
How B. infantis 35624 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 and failed — no benefit over placebo for general abdominal discomfort or bloating.
Standard live-probiotic caution applies; no strain-specific data exists either way.
No data — untested rather than shown safe. Discuss with a clinician.
In 19 healthy subjects, two weeks of this strain significantly increased methane (but not hydrogen) excretion on lactulose breath testing, and the number of 'abnormal' tests rose from 10 to 13 of 19. The study's authors suggest discontinuing probiotics before a lactulose breath test, since results may be altered.
Tip: Published trials did not quantify adverse-event rates; reported tolerability was consistently good
B. infantis 35624 has an evidence score of 5/10 — moderate evidence based on 13 indexed studies, including 2 meta-analyses. The probiotic strain behind Align, and the one most often called 'the evidence-based one' for IBS. The reality is thinner: four single-strain trials, two of them null, sharing overlapping authorship — and the large positive dose-ranging trial enrolled women only. The dose-response is genuinely inconsistent rather than a clean 'less is more'. Representative study: PMID 28166427.
The commonly studied dose of B. infantis 35624 is 1 x 10^8 CFU (100 million) daily — the dose that won the only head-to-head dose comparison; 1 x 10^10 CFU also beat placebo in a separate trial, so higher doses are not ruled out. Individual needs vary — start at the lower end of the range and adjust based on how you respond.
Timing is flexible for B. infantis 35624 — consistent daily use matters more than the time of day. No trial tested time of day or with-food versus fasted dosing.
B. infantis 35624 is generally well-tolerated and considered safe for most healthy adults at recommended doses. Reported side effects are uncommon and include 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.
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.
B. lactis (BB-12 / HN019)
Mostly mechanism / observationalThe most-replicated probiotic effect on gut transit — it measurably speeds things up and adds roughly one bowel movement a week in people who are already low. But it is not a constipation cure: the two largest, best-designed trials both missed their primary endpoints, and the honest effect is hours off transit time, not a fix.
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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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