Antiemetics: Drug Classes, Mechanisms, and Nursing Considerations

Antiemetics are central to managing nausea and vomiting from chemotherapy, postoperative recovery, motion sickness, and GI disorders. This guide reviews the major drug classes, their mechanisms, and the safety and monitoring concerns nursing students must master for exams and practice.

Antiemetic Drug Classes at a Glance

• 5-HT3 antagonists — Ondansetron (Zofran): block serotonin receptors in the GI tract and chemoreceptor trigger zone (CTZ). First-line for chemotherapy-induced nausea; monitor for QT prolongation.
• Dopamine antagonists — Metoclopramide (Reglan), Prochlorperazine: block D2 receptors at the CTZ. Cause EPS; also prokinetic; black box warning for tardive dyskinesia.
• Antihistamines — Meclizine, Diphenhydramine, Promethazine: block H1 receptors in the vestibular system. Used for motion sickness; cause significant sedation.
• Anticholinergics — Scopolamine: block muscarinic receptors in the vestibular system. Transdermal patch for motion sickness; contraindicated in glaucoma.
• NK1 antagonists — Aprepitant (Emend): block substance P at NK1 receptors. Used for delayed chemotherapy nausea; CYP3A4 interactions.
• Phenothiazines — Promethazine (Phenergan), Prochlorperazine: block dopamine and histamine receptors. Cause EPS and severe sedation; IV use can cause tissue necrosis.

Promethazine (Phenergan)

• A phenothiazine with antihistamine, antiemetic, and sedative properties used for nausea, motion sickness, and postoperative nausea.
• Causes significant sedation and anticholinergic effects: dry mouth, blurred vision, urinary retention, constipation.
• IV administration is dangerous — extravasation can cause severe tissue injury, necrosis, and gangrene.
• Deep IM injection is preferred. If IV is required, give through a large vein with close monitoring.
• May cause respiratory depression, especially in children or with other CNS depressants.
• Contraindicated in children under 2 years due to risk of fatal respiratory depression.
• Extrapyramidal symptoms (dystonia, akathisia) may occur and require discontinuation.
• Never administer intra-arterially — risk of severe arteriospasm and gangrene.

5-HT3 Receptor Antagonists

• Ondansetron is the most commonly used 5-HT3 antagonist; available as IV, oral, and orally disintegrating tablets.
• First-line for chemotherapy-induced nausea and vomiting (CINV); blocks serotonin receptors in the GI tract and CTZ.
• Constipation is the most common side effect.
• QT prolongation is a serious adverse effect — ECG monitoring in high-risk patients.
• Palonosetron is a second-generation agent with a longer half-life and better control of delayed chemotherapy nausea.

Dopamine Receptor Antagonists

• Metoclopramide blocks D2 receptors in the CTZ and has prokinetic effects that accelerate gastric emptying.
• Carries a black box warning for tardive dyskinesia with long-term use.
• EPS (dystonia, akathisia) require prompt recognition and discontinuation.
• Contraindicated in GI obstruction, hemorrhage, or perforation.
• Prochlorperazine is another dopamine-blocking antiemetic used for severe nausea.

Antihistamines and Anticholinergics

• Meclizine is used for motion sickness and vertigo; take about 1 hour before travel.
• Antihistamines commonly cause sedation and anticholinergic effects (dry mouth, urinary retention, blurred vision).
• Scopolamine is available as a transdermal patch behind the ear, lasting up to 72 hours.
• The patch must be removed before MRI — the aluminum backing can cause burns.
• Contraindicated in glaucoma and benign prostatic hyperplasia (BPH).

NK1 Receptor Antagonists

• Aprepitant blocks substance P at neurokinin-1 receptors in the brainstem; used for delayed chemotherapy-induced nausea.
• Often combined with ondansetron and dexamethasone for improved CINV control.
• Is a CYP3A4 inhibitor — can reduce effectiveness of oral contraceptives.
• Can decrease warfarin effectiveness — monitor INR.
• Common side effects: fatigue, hiccups, hypotension.

Exam Traps

• Ondansetron → QT prolongation; monitor cardiac status.
• Metoclopramide → EPS and tardive dyskinesia (black box).
• Promethazine IV → severe tissue injury and necrosis.
• Promethazine → contraindicated in children under 2 (respiratory depression).
• Antihistamines → avoid in glaucoma or BPH.
• Scopolamine patches → remove before MRI.
• Aprepitant → reduces oral contraceptive effectiveness.

Key Takeaways

• Ondansetron is first-line for chemotherapy-induced nausea but requires monitoring for QT prolongation.
• Metoclopramide blocks dopamine, accelerates gastric emptying, and carries a black box warning for tardive dyskinesia.
• Promethazine is highly sedating and can cause tissue necrosis with IV use — deep IM is preferred.
• Meclizine and scopolamine treat motion sickness via vestibular pathways but cause anticholinergic effects; scopolamine patches must be removed before MRI.
• Aprepitant targets substance P for delayed CINV and has important CYP3A4 interactions with oral contraceptives and warfarin.