PTCB Guide to Antiemetics

Also called anti-nausea and anti-vomiting drugs · suffix -setron and others · updated August 6, 2026

What gets tested

• Matching brand to generic: Zofran is ondansetron, Phenergan is promethazine, Reglan is metoclopramide, Compazine is prochlorperazine, Antivert is meclizine
• The receptor each drug blocks and which situation it suits
• The orally disintegrating tablet form of ondansetron and why it exists
• Promethazine's boxed warnings: tissue injury with injection and the under-2 contraindication
• Metoclopramide's tardive dyskinesia boxed warning and the duration limit that follows
• QT prolongation with ondansetron

What is not tested

You are not expected to build an antiemetic regimen for chemotherapy or to choose between agents for a patient. Focus on the brand and generic pairs, the receptor grouping, the dosage forms, and the boxed warnings and counseling points. Strengths, dosage forms, routes, special handling, and duration of therapy fall under PTCE knowledge area 1.4, which makes the orally disintegrating tablets, suppositories, and transdermal patches in this class very fair targets.

Why antiemetics matter

Ondansetron is widely dispensed and appears in almost every setting a technician might work in, from retail to oncology to the hospital floor. The class is also unusually rich in dosage forms: orally disintegrating tablets for a patient who cannot keep anything down, suppositories, transdermal patches, and injections, which makes it a natural vehicle for dosage form and route questions.

Antiemetics at a glance

Mechanism of action

Vomiting is coordinated by the brainstem, which gathers signals from several sources and acts on them through distinct receptor systems. That is why there is no single antiemetic: the useful drug depends on where the signal is coming from. The chemoreceptor trigger zone sits outside the blood-brain barrier and samples the blood for toxins, and it is dense in serotonin 5-HT3 and dopamine D2 receptors. Chemotherapy damages cells in the gut lining, which release large amounts of serotonin onto 5-HT3 receptors on the vagus nerve, which is why the -setron drugs work so well there and why they do comparatively little for motion sickness. Motion sickness originates in the vestibular system of the inner ear, which signals through histamine H1 and muscarinic acetylcholine receptors, so antihistamines such as meclizine and dimenhydrinate, and the anticholinergic scopolamine, are the agents that help. Dopamine blockers such as prochlorperazine and metoclopramide act at the chemoreceptor trigger zone, and metoclopramide adds a second useful action: it speeds gastric emptying, which is why it is used in gastroparesis. That same dopamine blockade is the reason metoclopramide and prochlorperazine can cause the movement side effects associated with antipsychotics, including tardive dyskinesia with prolonged metoclopramide use. Substance P acting on neurokinin-1 receptors drives the delayed phase of chemotherapy nausea, days after treatment, which is the niche aprepitant fills.

Common and important side effects

• Headache and constipation with the 5-HT3 blockers such as ondansetron
• QT interval prolongation with ondansetron, which is dose related
• Marked sedation with promethazine, dimenhydrinate, and meclizine
• Severe tissue injury including gangrene with injected promethazine, which carries a boxed warning
• Extrapyramidal symptoms with the dopamine blockers, and tardive dyskinesia with prolonged metoclopramide use, which carries a boxed warning
• Anticholinergic effects with scopolamine: dry mouth, blurred vision, urinary retention, and confusion in older adults
• Restlessness or akathisia with metoclopramide and prochlorperazine
• Low blood pressure and dizziness

Key interactions

• Other QT-prolonging drugs increase arrhythmia risk when combined with ondansetron
• Alcohol and other sedatives deepen the drowsiness of promethazine, meclizine, and dimenhydrinate
• Metoclopramide combined with an antipsychotic increases the risk of extrapyramidal symptoms, because both block dopamine
• Anticholinergic drugs add to the dry mouth, constipation, and confusion caused by scopolamine
• Metoclopramide speeds gastric emptying and can change the absorption of other oral drugs
• Serotonergic drugs combined with ondansetron carry a theoretical risk of serotonin syndrome

Patient counseling notes

• The orally disintegrating tablet dissolves on the tongue and does not need water, which is the point: it can be taken when the patient cannot keep liquids down. Handle it with dry hands and do not push it through the foil
• Apply the scopolamine patch behind the ear several hours before travel, and wash the hands after handling it, because rubbing the eye afterwards can blur vision
• Take motion sickness antihistamines before travel rather than after symptoms start
• Expect drowsiness with promethazine, meclizine, and dimenhydrinate, and avoid driving until the effect is known
• Report any involuntary movements of the face or tongue while taking metoclopramide, and do not continue it beyond the prescribed duration
• Report palpitations or fainting on ondansetron
• Sip clear fluids and take the dose before eating where directed, to get ahead of the nausea rather than chase it

Practice questions

Work the Antiemetics questions below first, then widen out. Two skills carry most of the Medications domain, and each has its own quiz. The first is drug class identification: recognising which class a drug belongs to from its name or its stem, which is what the drug class identification quiz drills across every class. The second is brand and generic recall, because the exam switches between the two names freely and expects you to follow. Pair this guide with the brand and generic quiz until the brand and generic pairs in the table above come back without effort.

Related drug-class guides

Official sources

• MedlinePlus: Ondansetron
• MedlinePlus: Promethazine
• MedlinePlus: Metoclopramide
• MedlinePlus: Prochlorperazine
• MedlinePlus: Meclizine
• MedlinePlus: Scopolamine Transdermal Patch
• ClinCalc DrugStats: Ondansetron (dispensing data)

PTCB Quiz Prep is an independent study resource, not affiliated with the Pharmacy Technician Certification Board. This guide is for exam preparation, not medical advice. Part of the PTCB Medications domain.