The purpose of this study is to evaluate the anti-emetic effect of oral ondansetron. For this evaluation, the investigators will perform a multi-center, double-blind, placebo-controled, randomized study. The investigators assumption is that oral ondansetron prior to intramuscular ketamine will reduce the occurrence of ketamine-induced vomiting.

Methods

• a multi-center, double-blind, placebo-controled, randomized study ( 4 tertiary hospital ED)
• survey for the occurrence of vomiting at ED and after discharge

Primary outcome

• the number of vomiting at ED and at home

Secondary outcome

• parent's satisfaction and ED length of stay

Inclusion Criteria:

• children under 18 years of age
• children who are planned to admit ketamine intramuscular injection
• children with ASA classification class I or II

Exclusion Criteria:

• don't want to enroll to study
• children with condition of contraindication of ketamine (IICP, IIOP, psychosis, brain tumor, thyroid disease, porphyria)
• children with condition of contraindication of ondansetron (hypersensitivity to 5-HT3 antagonist, Long QT syndrome, severe liver failure, obstructive ileus)
• children under 3 months of age
• children whose weight are over 30 kg
• children managed by other drug with ketamine