Ondansetron
A 5-HT3 receptor antagonist antiemetic that blocks serotonin in the GI tract and chemoreceptor trigger zone to prevent nausea and vomiting.
On the exam, ondansetron is usually a cardiac-safety question disguised as an antiemetic question. The “tell” is a patient with hypokalemia, hypomagnesemia, or a co-prescribed QT-prolonging drug (think agents like methadone, haloperidol, or a fluoroquinolone): the answer hinges on correcting potassium and magnesium and getting a baseline ECG before pushing IV doses, since a single IV dose is now capped at 16 mg to limit QT prolongation and torsades risk. When asked about a standard push, give the 4 mg dose slowly over 2–5 minutes (larger IV doses are infused over at least 15 minutes).
The classic trap is confusing ondansetron with the prokinetic metoclopramide. Ondansetron (a 5-HT3 blocker) does not speed GI motility — so don’t pick “increases GI motility,” and don’t pick the drug contraindicated in bowel obstruction; both belong to metoclopramide, which also causes extrapyramidal symptoms (dystonia, tardive dyskinesia). Also watch for serotonin syndrome when ondansetron is combined with SSRIs/SNRIs. Hook: “-setron stops the spew but stretches the QT.”
PlayPrepHQ study notes are written and reviewed against primary exam sources. How we create & review content →