Lactobacillus casei Shirota: What the Yakult Trials Actually Show

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Lactobacillus casei Shirota is the probiotic strain in Yakult, the Japanese fermented dairy drink that has been on the market since 1935. It is also one of the most-studied probiotic strains in the published literature, with over 60 years of human research behind it. The marketing claims are larger than the trial record supports — but the trial record itself is substantial and worth understanding strain by indication.

Indication 1: Acute Diarrhea Prevention in Children

The strongest pediatric evidence for Lactobacillus casei Shirota is in preventing community-acquired acute diarrhea. A 2010 Argentinian community trial in 638 healthy children showed that daily Yakult consumption over 90 days reduced acute diarrhea episode incidence versus control. A 2013 meta-analysis of 8 RCTs across multiple strains confirmed that probiotic supplementation reduced acute infectious diarrhea duration by roughly one day, with L. casei Shirota contributing meaningfully to the pooled effect. Effect sizes were modest but reproducible.

Indication 2: Constipation in Adults

Multiple Japanese and European RCTs have tested L. casei Shirota at 10¹⁰ CFU daily for chronic functional constipation. The strain modestly improves stool frequency and consistency over 2–4 weeks of daily intake, with effect sizes similar to mild osmotic laxatives but with no GI cramping signal. The largest trial in older adults found that 8 weeks of daily Yakult consumption improved Bristol stool scale scores and increased bowel movement frequency versus placebo. The effect is real but small — for severe chronic constipation, the strain is an adjunct, not a substitute for fiber and adequate hydration. See the constipation condition page.

Indication 3: Antibiotic-Associated Diarrhea

L. casei Shirota has been tested as a preventive against antibiotic-associated diarrhea. A 2007 trial in 135 hospitalized adults on broad-spectrum antibiotics showed reduced AAD incidence in the probiotic arm versus placebo, and substantially reduced C. difficile-associated diarrhea. The trial has been criticized for its design and was not consistently replicated in later AAD meta-analyses. The strain has weaker AAD evidence than Saccharomyces boulardii or Lactobacillus rhamnosus GG. See our probiotics + antibiotics piece.

Indication 4: Bladder Cancer Recurrence

A small Japanese RCT showed that daily L. casei Shirota intake reduced bladder cancer recurrence in patients post-transurethral resection of superficial bladder tumors. The signal is intriguing — proposed mechanisms include NK cell activation and reduced procarcinogen exposure — but the trial has not been replicated in larger Western cohorts and the strain should not be considered a cancer-prevention agent on the basis of one trial.

Dose, Form, and Practical Use

The Yakult bottle delivers approximately 10¹⁰ CFU of Lactobacillus casei Shirota per 65 mL serving, taken once daily. The same strain is available in capsule form in some markets at equivalent CFU counts. The carrier matrix matters less than the CFU dose; both deliver viable bacteria to the small intestine. As with all probiotics, take with or without food, refrigerate per label, and continue for 4+ weeks before judging effect. The strain has an excellent safety profile across decades of use, but immunocompromised adults (e.g., on chemotherapy, post-transplant) should not start any live probiotic without infectious-disease input. See our broader strain comparisons in L. rhamnosus GG, L. gasseri, and the gut-brain probiotic overview.

Bottom Line

Lactobacillus casei Shirota is a well-characterized probiotic with the strongest evidence for childhood diarrhea prevention and chronic adult constipation. It is not the best strain for IBS, mood, or weight — those are dominated by other strains. The Yakult marketing claims are broader than the trial evidence supports, but the underlying strain has more clinical study than most.