Benefits
Digestive support during antibiotic courses
In a meta-analysis of 63 trials (11,811 people), probiotics were linked to a 42 percent lower risk of antibiotic-associated diarrhea. A Cochrane review of 33 trials in children found 8 percent versus 19 percent, with doses of 5 billion CFU/day or more working better. Strains were poorly documented in many trials, so which products work best is unclear.
Gut support for people at high risk during antibiotic use
A Cochrane review of 31 trials (8,672 people) found C. difficile diarrhea in 1.5 percent of people taking probiotics with antibiotics versus 4.0 percent of controls. The benefit came from trials where baseline risk was above 5 percent, not from lower-risk trials, and probiotics did not reduce C. difficile detection in stool.
Immune support in upper respiratory tract trials
A Cochrane review of 23 trials (6,950 children, adults and older people) found probiotics reduced the number of people with at least one upper respiratory infection (risk ratio 0.76) and antibiotic use for these infections (0.58). Certainty was low to moderate, and most trials used one or two specific strains, so results do not apply to every blend.
Bowel comfort, depending on the strain
A meta-analysis of 82 trials (10,332 adults with IBS) found that some strains and combinations may ease overall symptoms, abdominal pain or bloating, but certainty was low to very low in almost every analysis. Side effects were not more common than with placebo. The American Gastroenterological Association recommends probiotics for IBS only within clinical trials.
Mechanism of action
Competitive exclusion
Probiotic microbes are thought to compete with harmful microbes for space and nutrients on the gut lining. In the Cochrane C. difficile review, probiotics lowered C. difficile diarrhea but did not reduce detection of C. difficile in stool, so the effect may not come from simply displacing it.
Short-chain fatty acid production
Gut bacteria ferment dietary fiber into short-chain fatty acids such as butyrate, propionate and acetate, which feed colon cells and help maintain the gut barrier. How much a given probiotic adds to this varies by strain.
Immune signaling
Probiotics interact with immune receptors on gut lining and immune cells, which is the proposed basis for effects on immune responses. These mechanisms come mostly from laboratory research.
Clinical trials
Cochrane review of 39 RCTs (9,955 adults and children) on probiotics for preventing Clostridioides difficile-associated diarrhea (CDAD). (Goldenberg et al. 2017, Cochrane Database Syst Rev)
Pooled across 39 RCTs; the CDAD outcome came from 31 trials (8,672 participants), adults and children.
In a complete-case analysis probiotics reduced CDAD by 60% (RR 0.40, 95% CI 0.30 to 0.52; moderate certainty), with CDAD incidence 1.5% on probiotics versus 4.0% on control. The benefit was concentrated in trials with baseline CDAD risk above 5% and was not seen at lower baseline risk. Detection of C. difficile in stool was not reduced. The review notes recent guidelines did not recommend probiotic prophylaxis.
Cochrane review of 33 randomized trials of probiotics given to children receiving antibiotics. (Guo et al. 2019, Cochrane Database Syst Rev)
6,352 children aged 0 to 18 years.
Antibiotic-associated diarrhea occurred in 8% of children on probiotics versus 19% of controls (RR 0.45; moderate certainty). Doses of 5 billion CFU/day or more were more effective than lower doses. No serious adverse events were reported in the 24 trials that tracked them. Risk of bias was high in 20 of the 33 studies.
Cochrane review of 23 randomized trials comparing probiotics with placebo or no treatment for preventing acute upper respiratory tract infections. (Zhao et al. 2022, Cochrane Database Syst Rev)
6,950 children, adults and older people.
Probiotics may reduce the number of people with at least one infection (RR 0.76; low certainty), the episode duration (by about 1.2 days; low certainty) and antibiotic use for these infections (RR 0.58; moderate certainty), without more adverse events. Most trials used one or two specific strains, not broad blends.
Systematic review and meta-analysis of randomized placebo-controlled trials of probiotics in adults with IBS. (Goodoory et al. 2023, Gastroenterology)
82 trials with 10,332 adults with IBS.
Some strains and combinations may improve global symptoms, abdominal pain or bloating, but certainty in the evidence was low to very low across almost all analyses, and only 24 trials were at low risk of bias. Adverse events were not more common with probiotics.