ObjectiveTo assess the efficacy of gelatine tannate (a complex of tannic acid with astringent and anti-inflammatory properties, and a protective gelatine) for the treatment of acute gastroenteritis (AGE) in children.DesignRandomised, double-blind, placebo-controlled trial. Intention-to-treat analysis.SettingTwo paediatric hospitals in Warsaw.ParticipantsChildren younger than 5 years of age with AGE, defined as a change in stool consistency to a loose or liquid form (according to the Bristol Stool Form Scale or Amsterdam Stool Form Scale) and/or an increase in the frequency of evacuations (≥3 in 24 hours), lasting for no longer than 5 days.InterventionsSeventy-two children were assigned to receive gelatine tannate (n=36) or placebo (n=36) in addition to standard rehydration therapy. The gelatine tannate was administered at an age-dependent dose (250–500 mg), and both study products were taken four times per day for 5 days.Primary and secondary outcome measuresThe main outcome measure was duration of diarrhoea. Secondary outcomes included the need for intravenous rehydration, need for hospitalisation of outpatients, number of watery stools per day, vomiting, weight gain, adverse events, recurrence of diarrhoea, severity of diarrhoea according to the Vesikari Scale and use of concomitant medications.ResultsSixty-four children (89%) completed the intervention and were included in the analysis. The duration of diarrhoea after randomisation was similar in the gelatine tannate and placebo groups (75.6±27.8 vs 75.5±29.0 hours, respectively, mean difference 0.1 hours, 95% CI −14.1 to 14.3 hours). There was no significant difference between groups in the number of watery stools per day throughout the study period. There were also no differences in any other secondary outcome measures between groups.ConclusionIn children with AGE younger than 5 years of age, gelatine tannate was ineffective as an adjunct to rehydration therapy.Trial registration numberNCT02280759.