Short answer: Probiotics are live microorganisms; prebiotics are mostly fibers, such as inulin, that gut microbes use as fuel. Products with both are called synbiotics.1 Their evidence is for different things: certain probiotics cut antibiotic diarrhea in children from 19% to 8% in a Cochrane review,2 while inulin raised gut bifidobacteria in 50 studies.3
| Probiotics | Prebiotics | |
|---|---|---|
| What they are | Live microorganisms that give a health benefit in adequate amounts1 | Substances gut microbes selectively use, giving a health benefit4 |
| Examples | Lactobacillus, Bifidobacterium, Saccharomyces boulardii1 | Inulin and other fructo-oligosaccharides1 |
| Measured in | CFU; many products 1 to 10 billion per dose1 | Grams; inulin trials used 3 to 20 g a day3 |
| Best-supported use | Preventing antibiotic diarrhea with certain strains21 | Raising bifidobacteria and bowel function (inulin)3 |
| Strain matters? | Yes; one strain's results do not carry over to another5 | Evidence is specific to each prebiotic4 |
| Safety | Usually minor; rare blood infections in very ill or immunocompromised people1 | Not described for the group in the sources checked |
Probiotics: a Cochrane review of 33 trials in 6,352 children taking antibiotics found diarrhea in 8% given probiotics versus 19% of controls, with moderate-certainty evidence.2 NIH says Lactobacillus rhamnosus GG cut antibiotic diarrhea from 22.4% to 12.3%, Saccharomyces boulardii from 17.4% to 8.2% in adults, and that starting within 2 days of the first antibiotic dose helped.1
Prebiotics: a 2023 meta-analysis of 50 studies (2,525 people) found chicory inulin-type fructans at 3 to 20 g a day raised bifidobacteria and improved bowel function in healthy people; two authors work for an inulin maker.3
We found no trial that compared a probiotic with a prebiotic for the same outcome.
NIH says probiotics are unlikely to harm healthy people, but they have been linked to rare blood infections, mostly in people who were severely ill or immunocompromised.1 In 2023 FDA warned that preterm infants given probiotics are at risk of severe, potentially fatal infections.5
More detail: Psyllium vs inulin: which fiber does what?, and the Probiotics and Prebiotics guides.