Probiotics During Antibiotics: Does the Strain Actually Matter?

“Take a probiotic with your antibiotics” sounds like one simple piece of advice.

The research is more interesting than that. Antibiotics can disrupt parts of the intestinal microbiome as they act against susceptible bacteria. One possible consequence is antibiotic-associated diarrhoea (AAD). Probiotics have therefore been studied as a way of reducing that risk.

Overall, the evidence is reasonably encouraging

A meta-analysis of 42 studies involving 11,305 adults found that probiotics taken alongside antibiotics reduced the risk of antibiotic-associated diarrhoea by about 37% overall. A newer 2025 meta-analysis of 15 randomized trials involving 7,427 participants reported a similar overall direction of effect.

“Probiotic” isn’t one ingredient

Different probiotics contain different organisms—and sometimes different strains of the same species. Reviews of strain specificity show that probiotic efficacy can be both strain-specific and condition-specific. A study showing benefit from one defined probiotic therefore cannot automatically prove that every bottle labelled “probiotic” will produce the same effect.

This is one reason the biggest CFU number on the shelf is not necessarily the most useful piece of information.

Timing may matter too

NIH consumer guidance notes that some probiotic strains may reduce antibiotic-associated diarrhoea, with benefit in research particularly when probiotics are started close to antibiotic initiation. But this should not become another universal internet rule. Age, health, the antibiotic being used, the reason for treatment and the actual probiotic organism all matter.

People who are seriously ill or immunocompromised should discuss probiotic use with their clinician rather than assuming that a live-microorganism supplement is automatically appropriate.

What should you look for on a probiotic label?

Instead of asking only, “How many billion CFU?”, also ask which organisms and strains are present, whether there is clinical evidence for those strains and the reason you are considering them, and whether the product guarantees viable organisms through the relevant shelf-life period.

A probiotic isn’t defined merely by having a very large number printed on its bottle. Its identity matters.

Further reading

• NIH Office of Dietary Supplements: Probiotics
• Why the Biggest CFU Number Isn’t Necessarily the Best Probiotic