However, its use has stagnated in many low- and middle-income countries (LMIC) where many children lack access to alternatives such as intravenous (IV) rehydration. When such children have fluid losses that cannot be replaced orally due to intractable vomiting, death is common. Finding a safe, non-invasive, and effective strategy to reduce vomiting in children would substantially decrease the need for IV rehydration and hence morbidity and mortality in LMICs. Although antiemetic agents are included in the WHO list of Essential Medicines, their use in children with gastroenteritis is not endorsed by the World Health Organization (WHO). Concerns include a lack of evidence that antiemetic agents can improve outcomes and that they are associated with dangerous side effects. However, in high-income settings, studies on ondansetron, an antiemetic agent, have demonstrated that it can reduce vomiting, IV rehydration, and hospitalization. Recent reviews by prominent organizations (e.g. International child Health Review Collaboration; the Committee on the Selection and Use of Essential Medicines) have indicated an interest in ondansetron use in children with gastroenteritis, and they have concluded that further evidence is required. This trial aims to determine if the administration of a single dose of oral ondansetron results in improved outcomes in children brought for emergency department care with vomiting and diarrhea in Pakistan.