# Antiemetic Drugs

> Antiemetic drugs for NEET-PG Pharmacology: 5-HT3 and NK1 antagonists, dopamine blockers, antihistamines, hyoscine and choosing drugs by cause of vomiting.

- Canonical URL: https://prepelephant.com/topics/neet-pg/pharmacology/antiemetic-drugs
- Exam / course: NEET-PG · Subject: Pharmacology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Antiemetic Drugs", PrepElephant, https://prepelephant.com/topics/neet-pg/pharmacology/antiemetic-drugs

## Direct answer

Antiemetics are chosen by the vomiting pathway involved: 5-HT3 receptor antagonists (ondansetron, palonosetran) dominate chemotherapy-induced and postoperative nausea through vagal and central serotonin receptors; NK1 antagonists (aprepitant) add control of delayed chemotherapy emesis; dopamine D2 blockers (metoclopramide, domperidone, phenothiazines) act on the chemoreceptor trigger zone and aid gastric emptying; antihistamines and antimuscarinics (dimenhydrinate, promethazine, hyoscine) suit vestibular and motion-related vomiting; and dexamethasone and benzodiazepines are valuable adjuncts in chemotherapy and anticipatory emesis respectively.

## What you must remember

- **5-HT3 antagonists:** ondansetron 8 mg intravenous or oral for chemotherapy, radiotherapy and postoperative nausea; adverse effects are constipation, headache and dose-dependent QT prolongation; palonosetron has a long half-life suited to delayed emesis.
- **NK1 antagonists:** aprepitant (oral) and fosaprepitant (intravenous) block substance B-mediated delayed emesis; aprepitant is a CYP3A4 inhibitor and substrate — adjust co-administered steroids and watch warfarin and statins.
- **Metoclopramide:** D2 antagonist and prokinetic (10 mg thrice daily, maximum five days) for gastroparesis-related vomiting; extrapyramidal reactions, especially acute dystonia in the young, are treated with diphenhydramine or benztropine; contraindicated in complete bowel obstruction and phaeochromocytoma.
- **Domperidone:** peripheral D2 blockade without crossing the blood-brain barrier, so minimal sedation or dystonia, but QT prolongation and galactorrhoea occur.
- **Vestibular and motion sickness:** H1 antihistamines (dimenhydrinate, cyclizine, promethazine) plus the antimuscarinic hyoscine (scopolamine) transdermal patch — the only class effective against motion sickness; ondansetron is weak here.
- **Adjuncts:** dexamethasone potentiates all antiemetic classes in chemotherapy and reduces vomiting from raised intracranial pressure; benzodiazepines prevent anticipatory vomiting; olanzapine is used in highly emetogenic or refractory settings.
- **Pregnancy:** doxylamine with pyridoxine is first-line for nausea of pregnancy, with promethazine or metoclopramide as second line and ginger or B6 as adjuncts.

## Common confusion

Ondansetron is often misapplied to motion sickness — vestibular vomiting responds to antihistamine-antimuscarinic drugs, not 5-HT3 blockade. The second distinction is metoclopramide versus domperidone: both are prokinetic D2 blockers, but metoclopramide enters the central nervous system (hence dystonia and sedation) while domperidone stays peripheral (hence QT risk instead of extrapyramidal risk).

## Exam-focused takeaway

Stems pair the emetic trigger with the right receptor class — cisplatin chemotherapy gets triple therapy (5-HT3 antagonist, dexamethasone, NK1 antagonist), a bus journey gets dimenhydrinate or hyoscine, gastroparesis gets metoclopramide or domperidone, and raised intracranial pressure gets dexamethasone. Acute dystonia in a young patient given metoclopramide and QT issues with ondansetron are favourite toxicity one-liners. Learn the four pathways — vagal serotonin, CTZ dopamine, vestibular histamine-acetylcholine, and central — and the topic collapses into a simple mapping.

## Frequently asked questions

### Which antiemetic class is preferred for chemotherapy-induced vomiting?

5-HT3 antagonists with dexamethasone, plus an NK1 antagonist like aprepitant for highly emetogenic regimens such as cisplatin.

### Why is ondansetron ineffective in motion sickness?

Motion emesis is mediated through vestibular acetylcholine and histamine pathways, which ondansetron does not block; hyoscine or antihistamines work instead.

### What is the main hazard of metoclopramide?

Extrapyramidal reactions including acute dystonic reactions and tardive dyskinesia, especially in young patients — hence the five-day use limit.

### How does domperidone differ from metoclopramide?

It does not cross the blood-brain barrier, avoiding central dystonia and sedation, but prolongs QT and raises prolactin causing galactorrhoea.

### What is the role of aprepitant and its interaction concern?

It blocks NK1 receptors for delayed chemotherapy emesis and inhibits CYP3A4, requiring dose adjustment of co-administered dexamethasone and warfarin monitoring.

### Which antiemetic is first-line in pregnancy?

Doxylamine-pyridoxine combination, supplemented if needed by promethazine or metoclopramide after the first trimester as clinically appropriate.