Why the specific strain matters more than the CFU number
Probiotic effects are largely strain-specific: benefits demonstrated for one strain do not transfer to a different species or even a different strain of the same species. Among the better-studied options, Lactobacillus rhamnosus GG and the yeast Saccharomyces boulardii have some of the strongest randomized-trial evidence, mainly for situations like antibiotic-associated and certain infectious diarrheas. A high colony-forming-unit (CFU) count on the label does not by itself signal effectiveness if the strain is unstudied.
Reading probiotic claims with appropriate caution
Evidence is strongest for a few defined uses and much weaker for broad claims about immunity, mood, or general 'gut health'. Quality also varies because supplements are loosely regulated, so labeled strain and viable dose are not always guaranteed.
This is general educational information, not medical advice or a dosing recommendation. Choice of strain and dose should be matched to a specific goal with a qualified clinician, and people who are immunocompromised, critically ill, or have central venous lines should be especially cautious with live-microbe products.