Benefits
Digestive comfort in women: dose-ranging trial
In IBS, B. infantis 35624 at 1 million / 100 million / 10 billion CFU once daily for 4 weeks was tested against placebo. Only 100 million CFU/day showed significant improvement in abdominal pain/discomfort, bloating/distention, sense of incomplete evacuation, straining, urgency, and passage difficulty. Critically, 1 million and 10 billion CFU showed no benefit: a narrow dose window. The trial report also noted that the 10 billion CFU capsules had significant formulation problems, so that arm may have failed for manufacturing reasons rather than biology. Under 5% of participants withdrew because of side effects. This was one industry-funded 4-week study in women only, and the later meta-analysis cited on this page did not confirm a benefit for the strain used on its own.
Claims across IBS subtypes are not established
The subtype breakdown behind this comes from a 2022 open-label study with no placebo group, so improvement within each subtype there cannot be separated from the placebo response and normal symptom fluctuation. The meta-analysis cited on this page did not analyse results by subtype. Subtypes matter to readers because many IBS patients shift between constipation- and diarrhea-predominant patterns or experience alternating symptoms.
Early pilot study, delivered in a milk drink
A pilot study delivering 10 billion CFU/day via a malted milk beverage reported beneficial effects, but with documented methodological limitations (inadequate randomization, baseline imbalances, underpowered design). It motivated the dose-ranging follow-up. Note the form and dose gap: this was a malted milk drink at 10 billion CFU, not a capsule at 100 million CFU, so it does not test what is sold on the shelf.
Real-world follow-up under the new strain name (Sabaté 2022)
One 2022 study followed 233 people taking one capsule a day for 30 days under the updated B. longum 35624 name, and reported lower symptom severity and better quality of life. It was a prospective open-label study with no placebo group and no randomization, and it was funded by the product supplier, so it cannot show that the capsule caused the change. It is not in the reference list on this page.
Immune and cytokine signals, mostly from studies outside IBS
This strain has been tested for effects on inflammation markers. Three small placebo-controlled studies in ulcerative colitis, chronic fatigue and psoriasis reported lower blood C-reactive protein and lower cytokines, and the early IBS pilot reported a shift in one cytokine ratio. None of those papers are in the reference list on this page, and the meta-analysis that is cited measured no inflammation marker at all. Treat this as an early mechanism signal rather than an established effect.
Strain-specificity vs generic B. infantis
The clinical evidence applies specifically to strain 35624 — generic 'B. infantis' or 'B. longum' supplements do not have equivalent evidence. Strain-specific effects on cytokine modulation, gut adherence, and dose-response are well-characterized for 35624 but cannot be assumed for other strains.
Visceral antinociception
This is animal work. Rats given the strain showed reduced sensitivity to gut pain signals in a laboratory model of visceral hypersensitivity. Animal results do not carry across to people on their own, and no reference for this work appears on this page.
Mechanism of action
Strain-specific immunomodulation
B. infantis 35624 shows distinct immunomodulatory activity compared to other B. infantis or B. longum strains. Generic strain-level claims do not apply — the clinical evidence base is specific to 35624.
Gut-immune axis systemic modulation
Studies of this strain have reported changes in blood inflammation markers, mostly in conditions other than IBS, and the early IBS pilot reported a shift in one cytokine ratio alongside its symptom results. That link is an association, not proof, and no study cited on this page measured any inflammation marker.
Visceral antinociception
Preclinical evidence supports visceral antinociceptive effects via gut-brain axis pain pathway modulation. This is the mechanistic basis for the abdominal pain/discomfort reduction observed in IBS.
Adherence to human epithelial cells
Strain 35624 demonstrates adherence to human intestinal epithelial cells, supporting transient colonization and the local-effect mechanisms during the 4-week treatment window.
Narrow dose window, and higher is not better
One trial found benefit at 100 million CFU/day and no benefit at either 1 million or 10 billion CFU/day. The trial report noted that the 10 billion CFU capsules had significant formulation problems, so calling this a proven hormesis or U-shaped curve goes further than the data allow. What is fair to say is that a higher CFU count is not automatically better here, and that the commercial 1 billion CFU dose was never tested against placebo in that study.
Survives gastric acidity
The strain survives gastric acid transit to reach the small intestine and colon viable — necessary for the local mechanisms to operate.
Clinical trials
Whorwell PJ and colleagues, 2006, American Journal of Gastroenterology 101:1581-1590, registered as NCT00135031. A multicentre double-blind randomised placebo-controlled dose-ranging study. This paper is not in the reference list below.
362 women with irritable bowel syndrome, randomised to one of three doses or placebo for 4 weeks.
Whorwell PJ et al. 2006, Am J Gastroenterol 101:1581-1590, NCT00135031. Large-scale multicenter double-blind randomized placebo-controlled dose-ranging study. 362 female subjects with Rome II IBS, 4 weeks of 1 million / 100 million / 10 billion CFU/day or placebo. Only 100 million CFU/day showed significant improvement across abdominal pain/discomfort, bloating/distention, incomplete evacuation, straining, urgency, and passage difficulty. 1 million and 10 billion CFU/day showed no benefit. <5% AE withdrawal. Industry-sponsored by Procter & Gamble. The trial report also noted significant formulation problems with the 10 billion CFU capsules, so the failure of that arm may be a manufacturing issue rather than a biological ceiling. This paper is not in the reference list on this page.
O'Mahony L and colleagues, 2005. A pilot study delivering the strain in a malted milk drink. This paper is not in the reference list below.
A small pilot group. The page notes the study had inadequate randomisation, baseline imbalances and was underpowered.
O'Mahony L et al. 2005. Pilot study via malted milk beverage delivery at 10 billion CFU/day. Reported beneficial effects but with documented methodological limitations: inadequate randomization, baseline imbalances, underpowered design. Motivated the subsequent dose-ranging clinical trial. The drink form and the 10 billion CFU dose do not match the capsule that is sold. This paper is not in the reference list on this page.
Sabate JM and colleagues, 2022, World Journal of Gastroenterology 28:732-744, reporting under the reclassified B. longum 35624 name. This paper is not in the reference list below.
Adults with irritable bowel syndrome, assessed for disease severity and quality of life.
Sabaté JM et al. 2022, World J Gastroenterol 28:732-744. A prospective open-label study of 233 people taking one capsule daily for 30 days, reported under the reclassified B. longum 35624 name. Symptom severity and quality of life improved from baseline, but there was no placebo group and no randomization, so it cannot confirm efficacy. It was supplier-funded and is not in the reference list on this page.