Oral Ondansetron use for gastroenteritis has become v popular in many emergency departments.  In 1 study of 18 EDs, where it was a standard in nearly half all cases, there was no overall improvement in rates of either intravenous rehydration (remained around 18%) or hospital admission. There was a small decrease in re-attendance rates.

There was also a wide variation between institutions: perhaps the problem is not using Ondansetron it correctly eg not giving it soon enough, or rushing into IV fluids before allowing the drug time to work

Same group looked at Ondansetron in diabetic children with vomiting, again, usage increased from 0 to 67%. Admission rates dropped from 62% to 49% between these eras, as did use of IV fluids, but Ondansetron had no independent benefit.

From Archimedes Blog.  (Freedman S et al. JAMA Pediatr 2014;168:321–29, see also Editorial)( (Leung J et al. J Pediatr 2014. doi.org/10.1016/j.jpeds.2014.10.020). )