Antiemetics: Receptors, Drugs & When to Use Them

Vomiting is triggered through five receptors feeding the brain’s vomiting centre and chemoreceptor trigger zone (CTZ). Each antiemetic blocks one of them — so the secret is to match the receptor to the cause of the nausea.

5-HT₃ antagonists — “the setrons”

Ondansetron, granisetron block serotonin at the CTZ and gut. The most effective drugs for chemotherapy-induced and post-operative nausea. Watch for constipation, headache and QT prolongation.

D₂ (dopamine) antagonists

• Metoclopramide — also prokinetic (speeds gastric emptying); risk of extrapyramidal effects and tardive dyskinesia, especially in the young.
• Domperidone — doesn’t cross the blood-brain barrier well, so fewer central effects; QT caution.
• Prochlorperazine — a phenothiazine, useful in vertigo and migraine.

H₁ antihistamines

Cyclizine, promethazine, cinnarizine act on the vestibular pathway — best for motion sickness and a common choice in pregnancy. Main effect: sedation.

Muscarinic antagonist

Hyoscine (scopolamine) — the classic motion-sickness patch; anticholinergic effects (dry mouth, blurred vision, urinary retention).

NK₁ antagonists

Aprepitant blocks substance P at NK₁ receptors — added to a 5-HT₃ antagonist and a steroid for delayed chemotherapy-induced nausea.

Match the drug to the cause

• Chemotherapy: 5-HT₃ antagonist + dexamethasone (+ aprepitant if high-risk).
• Post-operative: ondansetron ± dexamethasone.
• Motion sickness: hyoscine or an H₁ antihistamine.
• Pregnancy: antihistamine (e.g. cyclizine) first-line.
• Gastroparesis / reflux: metoclopramide or domperidone (prokinetic).

Exam tip: link the receptor to the setting — 5-HT₃ for chemo/post-op, H₁ or muscarinic for motion sickness, antihistamine in pregnancy, and remember metoclopramide causes extrapyramidal effects in young patients.

Five receptors, five drug families — once you map each antiemetic to its receptor and its clinical setting, choosing the right one becomes automatic.