Surgery & Perioperative/Antiemetic

Antiemetics

also: ondansetron · cyclizine · metoclopramide · prochlorperazine · PONV

Overview

Anti-sickness drugs grouped by receptor target — choosing by cause (the receptor) is the schema that gets tested.

Mechanism

Each blocks a different emetic receptor. 5-HT₃ antagonists (ondansetron) block serotonin at the chemoreceptor trigger zone/gut — best for chemo/PONV. D₂ antagonists (metoclopramide, domperidone, prochlorperazine) block dopamine ± prokinetic (gastric emptying). H1/antimuscarinic antihistamines (cyclizine) act on the vestibular/vomiting centre — motion/mechanical.

Indications

Post-operative nausea & vomiting (PONV)
Chemotherapy-induced; gastroenteritis; migraine; pregnancy (specific agents)

The agents

Ondansetron5-HT₃ antagonist

PONV/chemo; QT prolongation, constipation; avoid 1st-trimester (small cleft risk).

Cyclizineantihistamine/antimuscarinic

Motion/mechanical; antimuscarinic effects; caution in heart failure.

MetoclopramideD₂ antagonist (prokinetic)

Avoid in young women/Parkinson's — extrapyramidal/dystonia; bowel obstruction CI.

ProchlorperazineD₂ antagonist

Vertigo/vestibular; extrapyramidal effects.

Adverse effects

Metoclopramide/prochlorperazine: extrapyramidal reactions (acute dystonia)serious

Esp. young women; treat with procyclidine.

Ondansetron: QT prolongation, constipationcommon

Cyclizine: anticholinergic (dry mouth, tachycardia)common

Cautions & contraindications

Metoclopramide in Parkinson's disease / bowel obstructionall

D₂ blockade worsens PD; prokinetic dangerous in obstruction.

Metoclopramide in young adults (dystonia risk)all

Interactions

Ondansetron + other QT-prolonging drugs
Additive sedation (cyclizine)

Monitoring & kinetics

Response; ECG if QT risk (ondansetron)

Choose by mechanism/cause. Oral/IV/IM.

Source: BNF — Antiemetics