Introduction Administration of some probiotics appears to reduce the risk of antibiotic-associated diarrhoea (AAD). The effects of probiotics are strain-specific, thus, the efficacy and safety of each probiotic strain should be established separately. We aim to assess the effects of Lactobacillus reuteri DSM 17938 administration for the prevention of diarrhoea and AAD in children.

Methods and analysis A total of 250 children younger than 18 years treated with antibiotics will be enrolled in a double-blind, randomised, placebo-controlled trial in which they will additionally receive L. reuteri DSM 17938 at a dose 10⁸ colony-forming units or an identically appearing placebo, orally, twice daily, for the entire duration of antibiotic treatment. The primary outcome measures will be the frequencies of diarrhoea and AAD. Diarrhoea will be defined according to 1 of 3 definitions: (1) ≥3 loose or watery stools per day for a minimum of 48 hours during antibiotic treatment; (2) ≥3 loose or watery stools per day for a minimum of 24 hours during antibiotic treatment; or (3) ≥2 loose or watery stools per day for a minimum of 24 hours during antibiotic treatment. AAD will be diagnosed in cases of diarrhoea, defined clinically as above, caused by Clostridium difficile or for otherwise unexplained diarrhoea (ie, negative laboratory stool tests for infectious agents).

Ethics and dissemination The Bioethics Committee approved the study protocol. The findings of this trial will be submitted to a peer-reviewed paediatric journal. Abstracts will be submitted to relevant national and international conferences.

Trial registration number NCT02871908.