Antiemetics

Mechanism-based prescribing — ondansetron, cyclizine & metoclopramide

The best antiemetic targets the cause of nausea. Ondansetron for post-op/chemo. Metoclopramide for gastric stasis (not Parkinson's, not obstruction). Cyclizine for vestibular/raised ICP. Haloperidol for end-of-life.

What to do

1 · Identify the cause

Post-op / chemo: ondansetron.

Opioid-induced / gastric stasis: metoclopramide.

Vestibular / motion: cyclizine.

Raised ICP: cyclizine (not metoclopramide).

End-of-life: haloperidol.

2 · Check drug chart

Is any antiemetic already prescribed? Has it been given? Is it the right drug for the cause? Avoid duplicating mechanisms.

3 · Choose agent

Cyclizine 50mg PO/IV/IM TDS.

Ondansetron 4–8mg PO/IV TDS.

Metoclopramide 10mg PO/IV TDS (max 5 days).

Haloperidol 500mcg–1.5mg PO/SC BD.

4 · Route selection

PO if tolerating fluids. IV if vomiting with IV access. IM if no IV access. SC for end-of-life. Avoid IM in anticoagulated patients.

5 · Check contraindications

Metoclopramide: not in Parkinson's, obstruction, epilepsy.

Ondansetron: caution in prolonged QT.

Cyclizine: anticholinergic — caution in prostatic hypertrophy, glaucoma.

6 · Reassess in 24 hours

Is the underlying cause improving? Step down to oral once tolerating. Persistent symptoms: consider different agent or investigate further.

Drugs

Drug	Dose	Route	Notes
Cyclizine	50mg	PO/IV/IM TDS	H1 antihistamine. Vestibular, motion, raised ICP. Can cause sedation.
Ondansetron	4–8mg	PO/IV TDS	5HT3 antagonist. Post-op, chemo, radiation. Caution: QT prolongation.
Metoclopramide	10mg	PO/IV TDS	D2 antagonist + prokinetic. Max 5 days. Contraindicated: Parkinson's, obstruction.
Haloperidol	500mcg–1.5mg	PO/SC BD	End-of-life nausea/agitation. Low dose. Useful in renal failure (unlike metoclopramide).

Red flags

Persistent vomiting despite antiemetics — investigate cause, consider ileus, bowel obstruction, raised ICP
Aspiration risk — consider NGT, nil by mouth, antiemetic by SC/IV route
Acute dystonic reaction (oculogyric crisis) — procyclidine 5mg IV/IM immediately
Severe headache + vomiting — raised ICP, do not give metoclopramide

Reference: BNF / NICE CKS Nausea and Vomiting / SIGN Guidelines