Gastroenterology

Antiemetics - Ondansetron, Metoclopramide, Promethazine song

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Antiemetics - Ondansetron, Metoclopramide, Promethazine

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Ondansetron is a 5-HT3 receptor antagonist that blocks serotonin signaling in the gut and chemoreceptor trigger zone (CTZ), making it first-line for chemotherapy-induced and postoperative nausea and vomiting.
Ondansetron can prolong the QT interval; it is contraindicated with other QT-prolonging agents and requires caution in patients with congenital long QT syndrome.
Metoclopramide is a D2 receptor antagonist that blocks the CTZ and enhances gastric motility (prokinetic), making it useful for both nausea and diabetic gastroparesis.
Long-term metoclopramide use carries a black box warning for tardive dyskinesia, an irreversible movement disorder caused by chronic dopamine receptor blockade in the nigrostriatal pathway.
Metoclopramide can cause acute extrapyramidal symptoms (dystonia, akathisia) that are treated with diphenhydramine or benztropine.
Promethazine is a first-generation H1 antihistamine with strong anticholinergic and D2 antagonist properties, used for nausea, motion sickness, and allergic conditions.
Promethazine carries a black box warning against IV use in children under 2 years due to risk of fatal respiratory depression; the preferred route in pediatrics is oral or rectal.
Motion sickness is best prevented by scopolamine (muscarinic antagonist) or antihistamines like meclizine, which target the vestibular pathway rather than the CTZ.
Metoclopramide is contraindicated in Parkinson's disease and bowel obstruction; D2 blockade worsens parkinsonian symptoms, and prokinetic activity is dangerous with mechanical obstruction.
Dexamethasone is commonly added to ondansetron for enhanced antiemetic prophylaxis in chemotherapy regimens, acting synergistically through an incompletely understood mechanism.