GradedHealth

Probiotics

Last reviewed October 2026 · Based on 25 studies

Bottom line

Some probiotics may help prevent diarrhoea caused by antibiotics, with a smaller benefit against C. difficile diarrhoea. For irritable bowel syndrome, a 2026 meta-analysis found symptom scores improved but quality of life did not clearly change, with low certainty. For children's abdominal pain, ventilated ICU patients and general gut health, trial results are weak or conflicting. Serious bloodstream infections from probiotic strains are rare but documented in very ill patients and children with central lines.

What it helps with

  • Prevents diarrhoea caused by antibiotics — Benefit, evidence grade C (Limited). Two meta-analyses of randomised trials: one rates moderate certainty, the 2025 Cochrane review low (few events, publication bias); downgraded for inconsistency across strains and doses and for publication bias.
  • Prevents C. difficile diarrhoea — Benefit, evidence grade C (Limited). Two Cochrane meta-analyses of 31 to 38 trials; downgraded for publication bias and missing data, and for subgroup inconsistency (benefit seen only in trials with higher baseline risk).
  • Eases irritable bowel syndrome symptoms — Benefit, evidence grade C (Limited). Several meta-analyses of 40 to 82 trials; downgraded for inconsistency between strains, low certainty in the 2026 review, and high risk of bias in many trials.
  • Improves quality of life in irritable bowel syndrome — Mixed results, evidence grade C (Limited). Pooled meta-analysis of 40 trials found no significant change; one 200-person trial reported gains; downgraded for inconsistency and low certainty.
  • Eases functional abdominal pain in children — Benefit, evidence grade C (Limited). Cochrane review of 18 trials; downgraded for inconsistency and risk of bias, and complete pain resolution was not clearly different.
  • Reduces ventilator-associated pneumonia in ICU patients (results conflict) — Mixed results, evidence grade C (Limited). One large trial found no effect and a meta-analysis found a benefit with low certainty; downgraded for inconsistency and risk of bias.
  • Lowers death risk in critically ill patients — No effect, evidence grade B (Moderate). One meta-analysis of 65 trials and one large trial agree; moderate certainty; downgraded for risk of bias in many included trials.
  • Rarely causes bloodstream infection in critically ill people and children with central lines — Harm, evidence grade C (Limited). Observational cases, one case report and a sterile-site signal in one trial; small numbers and indirect evidence, so downgraded.
  • Improves gut microbiome diversity in healthy adults — No effect, evidence grade C (Limited). Meta-analysis of 26 small trials measuring microbiome markers; surrogate outcome, downgraded for inconsistency across products.

What it is

Probiotics are live microbes given in amounts meant to benefit health [1]. Common examples are Lactobacillus rhamnosus GG, a bacterium, and Saccharomyces boulardii, a yeast [2][3][4].

What the research says

The most consistent evidence is for preventing antibiotic-associated diarrhoea: 42 adult trials found a 37% lower risk (risk ratio 0.63) [5], and a 2025 Cochrane review of 40 studies found the same direction (risk ratio 0.67) with low-certainty evidence [1]. For C. difficile diarrhoea, a 2025 Cochrane review of 38 trials found a risk ratio of 0.50 and an absolute reduction of 1.6%, with low-certainty evidence [1]. In irritable bowel syndrome, a 2026 meta-analysis of 40 trials found symptom scores improved by about 33 points (95% range 11 to 56), with no clear change in quality of life and low certainty [6]. An earlier review of 82 trials rated certainty low to very low for most analyses [7]. Results differ by strain: one network meta-analysis ranked Lactobacillus acidophilus DDS-1 highest for symptom severity [8], and another ranked Bacillus coagulans highest for symptom relief [9]. Single trials report symptom gains with specific products, such as a 307-person trial of Lactiplantibacillus plantarum Lpla33 [10]. In one large ICU trial, Lactobacillus rhamnosus GG did not reduce ventilator-associated pneumonia (21.9% vs 21.3%) [3]. A 2025 meta-analysis of healthy adults found more Lactobacillus in the gut with synbiotics, but no change in microbiome diversity or CRP and IL-6 [11].

Doses used in studies

In the children's antibiotic-diarrhoea review, the subgroup that worked best used 5 billion CFU or more a day [12]. The critical care trial gave Lactobacillus rhamnosus GG at 1 x 10^10 colony-forming units twice daily [3]. In irritable bowel syndrome, the Lpla33 trial gave 1 x 10^9 or 1 x 10^10 colony-forming units a day for eight weeks [10].

Safety and interactions

Common side effects in trials were abdominal cramping, nausea, fever, soft stools, flatulence and taste disturbance, in both probiotic and control groups [1]. Adult trials found no significant increase in adverse events [13]. In pregnancy and breastfeeding, a review found no serious concerns, but the evidence is thin and one study reported vaginal discharge and stool changes [14]. In that ICU trial, Lactobacillus rhamnosus was found in a sterile site or as the main organism in 1.1% of probiotic recipients versus 0.1% of controls [3]. In a 2026 case series of children after cardiac surgery in China, six cases of bacteraemia with probiotic strains occurred among 16,436 children, all with central venous catheters [15]. Saccharomyces fungaemia has been linked to S. boulardii use in intensive care patients [4]. The abstracts read set no upper safe dose and report no medicine interactions, and do not cover people with kidney or liver disease [1][12]. People who are immunocompromised or critically ill should ask a doctor first [1][3].

Caveats and open questions

Results depend on the strain, dose and the patient's baseline risk, so results cannot simply be applied to every product [7][5]. Several large reviews had authors or funding linked to probiotic companies [1]. Many trials are small or at high risk of bias [16][17]. For irritable bowel syndrome, the 2026 meta-analysis says more high-powered trials are needed [6].

References

  1. Probiotics for the prevention of Clostridioides difficile-associated diarrhea in adults and children.. Cochrane Database Syst Rev, 2025.
  2. Probiotics for the Prevention of Antibiotic-Associated Diarrhea in Children.. J Pediatr Gastroenterol Nutr, 2016.
  3. Effect of Probiotics on Incident Ventilator-Associated Pneumonia in Critically Ill Patients: A Randomized Clinical Trial.. JAMA, 2021.
  4. Epidemiology of Saccharomyces fungemia: A systematic review.. Med Mycol, 2023.
  5. Probiotics for the prevention of antibiotic-associated diarrhoea: a systematic review and meta-analysis.. BMJ Open, 2021.
  6. Probiotics for Irritable Bowel Syndrome: An Updated Systematic Review and Meta-Analysis With Trial Sequential Analysis.. J Gastroenterol Hepatol, 2026.
  7. Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis.. Gastroenterology, 2023.
  8. Outcome-Specific Efficacy of Different Probiotic Strains and Mixtures in Irritable Bowel Syndrome: A Systematic Review and Network Meta-Analysis.. Nutrients, 2023.
  9. Efficacy of Probiotics for Irritable Bowel Syndrome: A Systematic Review and Network Meta-Analysis.. Front Cell Infect Microbiol, 2022.
  10. Efficacy and dose response of Lactiplantibacillus plantarum in diarrhea-predominant irritable bowel syndrome.. World J Gastroenterol, 2023.
  11. The effect of oral synbiotics on the gut microbiota and inflammatory biomarkers in healthy adults: a systematic review and meta-analysis.. Nutr Rev, 2025.
  12. Probiotics for the prevention of pediatric antibiotic-associated diarrhea.. Cochrane Database Syst Rev, 2019.
  13. Probiotics for the Prevention of Antibiotic-associated Diarrhea in Adults: A Meta-Analysis of Randomized Placebo-Controlled Trials.. J Clin Gastroenterol, 2021.
  14. Are Probiotics and Prebiotics Safe for Use during Pregnancy and Lactation? A Systematic Review and Meta-Analysis.. Nutrients, 2021.
  15. Case Series of Bacteremia Associated with Probiotic Use in Children after Cardiac Surgery, China.. Emerg Infect Dis, 2026.
  16. Probiotics in Critical Illness: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.. Crit Care Med, 2022.
  17. Probiotics for management of functional abdominal pain disorders in children.. Cochrane Database Syst Rev, 2023.