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Do probiotics actually work? What the trials show

Some probiotics may reduce antibiotic-linked diarrhoea and C. difficile diarrhoea. Evidence for gut health, IBS and intensive care is weak or conflicting.

Published October 2026

Short answer

Some probiotics may help prevent antibiotic-linked diarrhoea, and C. difficile diarrhoea to a smaller degree. The evidence is limited, and other uses rest on weak, mixed or conflicting trials. Evidence grade C (Limited). Meta-analyses of randomised trials; one rates the antibiotic-diarrhoea result moderate certainty and the 2025 Cochrane review low, so downgraded for inconsistency and publication bias; other claims grade C or lower.

Key takeaways

  • Across 42 adult trials, probiotics given with antibiotics cut antibiotic-linked diarrhoea by 37% [1], though a 2025 Cochrane review rates this evidence as low certainty [2].
  • For C. difficile diarrhoea the benefit is real but small, and the certainty of the evidence is low [2].
  • In irritable bowel syndrome, some strains may help, but certainty is low to very low for almost all of them [3].
  • In one large ICU trial, a Lactobacillus rhamnosus GG probiotic did not reduce ventilator-associated pneumonia [4].
  • Bloodstream infections from probiotic strains are rare, but they are documented in very ill people and in children with central lines [4][5].

The short version

Probiotics are live microbes given in amounts meant to benefit health [2]. The most consistent evidence is for preventing diarrhoea caused by antibiotics, though one review rates its certainty moderate and another low [1][2]. The evidence for C. difficile diarrhoea is smaller [2]. For irritable bowel syndrome, general gut health and critical illness, the evidence is weak, mixed or conflicting [3][6][7]. Serious infections are rare, but they have been reported in very ill people [4][5].

What the studies show

Antibiotic-associated diarrhoea. A 2021 meta-analysis of 42 randomised trials (11,305 adults) found that probiotics given with antibiotics cut the risk of diarrhoea by 37% (risk ratio 0.63), and rated the evidence moderate [1]. A 2025 Cochrane review of 40 studies in adults and children found the same direction (risk ratio 0.67), but rated the evidence low certainty because of few events and possible publication bias [2]. A 2019 Cochrane review of children reported diarrhoea in 8% of the probiotic group compared with 19% of controls [8].

C. difficile diarrhoea. A 2025 Cochrane review of 38 trials (13,179 participants) found that probiotics reduced C. difficile diarrhoea (risk ratio 0.50), an absolute reduction of 1.6% [2]. The review rated this evidence as low certainty and said that one case may be prevented for every 65 people taking probiotics [2]. An earlier Cochrane review found, in a post hoc analysis, that the benefit appeared only in trials where the usual risk of C. difficile was above 5% [9].

Irritable bowel syndrome. A 2023 meta-analysis of 82 trials (10,332 adults) found a benefit for overall symptoms from some strains, but the certainty of evidence was low to very low for almost all of them [3]. A 2018 review reached a similar view: particular strains may help, but definite conclusions were not possible [10].

Children with abdominal pain. In a 2023 Cochrane review, six trials of 554 children with functional abdominal pain found treatment success in 50% of the probiotic group versus 33% of placebo, but the evidence is low certainty [11]. Complete resolution of pain did not clearly differ from placebo [11].

Critical illness. A large ICU trial randomised 2,653 ventilated adults to Lactobacillus rhamnosus GG or placebo. Ventilator-associated pneumonia occurred in 21.9% of the probiotic group and 21.3% of controls [4]. A 2022 meta-analysis of 65 trials (8,483 patients) found fewer pneumonias overall, but rated the evidence low certainty, and in post hoc analyses the effect disappeared when high-risk-of-bias studies were removed [6]. Neither source found a clear effect on death, which the meta-analysis rated as moderate certainty [6][4].

Healthy adults. A 2025 meta-analysis of 26 trials (1,319 healthy adults) found that synbiotics, which combine probiotics and prebiotics, increased Lactobacillus counts in the gut. It found no significant change in microbiome diversity or in inflammation markers such as CRP and IL-6 [7].

The nuances

Strains and doses matter. Products differ in species, strain and dose, and that makes it hard to generalise between them [3]. In the 2019 children's review, high doses of 5 billion CFU or more a day were more effective than low doses in a subgroup analysis [8]. A 2016 expert panel recommended Lactobacillus rhamnosus GG or Saccharomyces boulardii for children at risk of antibiotic diarrhoea, and said there was not enough evidence for other strains [12]. The adult review found the effect in studies with moderate or high baseline risk, but not in studies where the baseline risk was low [1].

Industry ties. The 2025 Cochrane review noted that authors had links to a probiotic company, and that probiotic companies funded 28 of the included studies [2].

Safety. In both groups of the C. difficile trials, the most common side effects were abdominal cramping, nausea, fever, soft stools, flatulence and taste disturbance [2]. A 2021 meta-analysis of adult placebo-controlled trials found no significant increase in adverse events (risk ratio 1.00) [13]. A 2021 review of pregnancy and breastfeeding found no serious health concerns for mother or infant, but one study reported more vaginal discharge and stool changes with Lactobacillus rhamnosus and L. reuteri [14]. Only 11 of the 100 eligible studies in that review reported adverse effects in a form it could analyse, so this evidence is thin [14].

Immunocompromised people and central lines. The 2025 Cochrane review says short-term use is likely not associated with adverse effects in people on antibiotics who are not immunocompromised [2]. In that ICU trial, Lactobacillus rhamnosus was found in a sterile site or as the main organism in 1.1% of the probiotic group versus 0.1% of controls [4]. A 2026 case series of children after cardiac surgery in China found six cases of bacteraemia with probiotic strains among 16,436 children. All six had central venous catheters, and three had not received probiotics directly, which the authors link to possible cross-contamination. The authors describe such infections as rare and usually resolved by antibiotics [5]. A 2022 case report described a Lactobacillus rhamnosus GG bloodstream infection in a patient with a central line, who was immunocompetent and cleared it without antibiotics [15]. A 2023 review of Saccharomyces fungaemia found that 73 of 108 cases had received S. boulardii, and that intensive care and feeding lines were the most common risk factors among them [16]. The review's all-cause mortality was 36.1%, with no significant difference between groups [16].

Gaps. The abstracts I read set no safe upper dose and do not report interactions with medicines, so this article cannot say whether any exist [2][8]. They also do not report results for people with kidney or liver disease [2].

What this means for you

These studies describe what was tested in trials, not whether a product will help a given person. People who are on antibiotics, have a central line, a weakened immune system or a serious illness should talk to a doctor before taking a probiotic [2][4].

Go deeper

  • Probiotics — Some probiotics may help prevent diarrhoea caused by antibiotics, with a smaller benefit against C.

References

  1. Probiotics for the prevention of antibiotic-associated diarrhoea: a systematic review and meta-analysis.. BMJ Open, 2021.
  2. Probiotics for the prevention of Clostridioides difficile-associated diarrhea in adults and children.. Cochrane Database Syst Rev, 2025.
  3. Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis.. Gastroenterology, 2023.
  4. Effect of Probiotics on Incident Ventilator-Associated Pneumonia in Critically Ill Patients: A Randomized Clinical Trial.. JAMA, 2021.
  5. Case Series of Bacteremia Associated with Probiotic Use in Children after Cardiac Surgery, China.. Emerg Infect Dis, 2026.
  6. Probiotics in Critical Illness: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.. Crit Care Med, 2022.
  7. The effect of oral synbiotics on the gut microbiota and inflammatory biomarkers in healthy adults: a systematic review and meta-analysis.. Nutr Rev, 2025.
  8. Probiotics for the prevention of pediatric antibiotic-associated diarrhea.. Cochrane Database Syst Rev, 2019.
  9. Probiotics for the prevention of Clostridium difficile-associated diarrhea in adults and children.. Cochrane Database Syst Rev, 2017.
  10. Systematic review with meta-analysis: the efficacy of prebiotics, probiotics, synbiotics and antibiotics in irritable bowel syndrome.. Aliment Pharmacol Ther, 2018.
  11. Probiotics for management of functional abdominal pain disorders in children.. Cochrane Database Syst Rev, 2023.
  12. Probiotics for the Prevention of Antibiotic-Associated Diarrhea in Children.. J Pediatr Gastroenterol Nutr, 2016.
  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. Bacteraemia caused by Lactobacillus rhamnosus given as a probiotic in a patient with a central venous catheter: a WGS case report.. Infect Prev Pract, 2022.
  16. Epidemiology of Saccharomyces fungemia: A systematic review.. Med Mycol, 2023.