Acute gastroenteritis (AGE) is one of the most common diseases among children. Oral rehydration therapy is the key treatment. According to the 2014 guidelines developed by the ESPGHAN probiotics may be considered in the management of children with AGE in addition to rehydration therapy. Considering that evidence on L reuteri remains limited, the investigators aim to assess the efficacy of L reuteri DSM 17938 for the treatment of AGE in children. Children vaccinated and not vaccinated against rotavirus will be evaluated separately.

Two independent reports (rotavirus-vaccinated and non-vaccinated children) are planned.

Inclusion Criteria:

1. Acute gastroenteritis (AGE) defined as a change in stool consistency to loose or liquid form (according to the Bristol Stool Form (BSF) scale or in the case of infants, the Amsterdam Stool Form (ASF) scale) and/or an increase in the frequency of evacuations (typically ≥3 in 24 h), lasting for no longer than 5 days.
2. Age: older than 1 month and younger than 60 months.
3. A caregiver must provide written informed consent.

Exclusion Criteria:

1. Use of antibiotics within two weeks prior to enrolment.
2. Use of gelatine tannate, diosmectite, probiotics, racecadotril, or zinc (including zinc containing ORS) within a week prior to enrolment (a single dose is allowed).
3. Breast feeding (>50%)
4. Chronic diarrhoeal gastrointestinal disease (e.g., inflammatory bowel disease, cystic fibrosis, coeliac disease, food allergy)
5. Immunodeficiency
6. Malnutrition (weight/height/length under 3rd percentile) (WHO Child Growth Standards will be used)