Anti-emetic agents: complete pharmacology and clinical use
Antiemetics prevent or relieve nausea and vomiting by blocking neurotransmitter pathways within the gastrointestinal tract, vestibular apparatus, chemoreceptor trigger zone (CTZ), nucleus tractus solitarius (NTS) and vomiting centre. Correct drug choice follows the likely cause: an H1/muscarinic drug for motion sickness is not the best choice for chemotherapy-induced vomiting, and a dopamine antagonist may be hazardous in Parkinson disease.
Learning objectives
- Map emetic pathways to their major receptors and drug targets.
- Classify anti-emetics and compare individual medicines within each class.
- Use adult dose tables safely, including important route and interval details.
- Recognise QT prolongation, extrapyramidal reactions, sedation, anticholinergic toxicity and serotonin-related interactions.
- Plan treatment for motion sickness, postoperative nausea and vomiting (PONV), chemotherapy-induced nausea and vomiting (CINV), gastroenteritis, migraine, gastroparesis and pregnancy-related vomiting.
Neurophysiology: where anti-emetics work
| Site | Important signals | Clinical implication |
|---|---|---|
| Gut mucosa and vagal afferents | 5-HT3, vagal and enteric pathways | Cytotoxic drugs, radiation and mucosal irritation release serotonin from enterochromaffin cells; 5-HT3 blockers are valuable in CINV and PONV. |
| CTZ / area postrema | D2, 5-HT3, NK1, opioid and other signals | Located outside the blood-brain barrier; detects drugs, toxins, uraemia and metabolic abnormalities. |
| NTS and vomiting centre | 5-HT3, NK1, H1, M1, D2 | Integrates gut, pharyngeal, vestibular and higher-cortical inputs. |
| Vestibular system | H1 and M1 | Motion sickness responds best to antihistamines or antimuscarinics. |
Classification at a glance
5-HT3 antagonists
Ondansetron, granisetron, palonosetron. Best known for CINV, radiotherapy-related vomiting and PONV.
Dopamine D2 antagonists
Metoclopramide, domperidone, prochlorperazine, chlorpromazine, droperidol and haloperidol.
H1 antihistamines
Promethazine, cyclizine, dimenhydrinate, meclizine. Especially useful in vestibular nausea and motion sickness.
Antimuscarinics
Hyoscine/scopolamine. Prevents motion sickness and can help prevent PONV.
NK1 antagonists
Aprepitant, fosaprepitant, netupitant. Central to high-emetic-risk chemotherapy prophylaxis.
Adjuvant agents
Dexamethasone, olanzapine, cannabinoids and benzodiazepines in selected settings.
1. 5-HT3-receptor antagonists (“setrons”)
Drugs: ondansetron, granisetron, palonosetron and, in some settings, dolasetron. They block 5-HT3 receptors on vagal afferents in the gut and centrally in the CTZ/NTS. They are highly effective for acute chemotherapy-related nausea and vomiting and for PONV, but are relatively weak for motion sickness because the vestibular pathway is mainly H1/M1-mediated.
| Drug | Common adult teaching dose | Useful indications | Key precautions |
|---|---|---|---|
| Ondansetron | 4 mg IV/IM for PONV; 4–8 mg PO/IV every 8–12 h when indicated. CINV regimens are protocol-specific; label-based injection prophylaxis commonly uses 0.15 mg/kg IV for 3 doses (maximum 16 mg/dose). | PONV, CINV, radiotherapy-related vomiting, selected acute-care nausea. | QT prolongation, constipation, headache; correct hypokalaemia/hypomagnesaemia; caution with other QT-prolonging drugs and in congenital long-QT syndrome. |
| Granisetron | 1–2 mg PO once daily or 1 mg IV before chemotherapy (protocol dependent). | CINV and PONV. | Class QT risk, headache, constipation. |
| Palonosetron | 0.25 mg IV once about 30 min before chemotherapy; longer half-life supports delayed CINV prevention. | CINV, especially delayed vomiting in protocol-based regimens. | Headache, constipation; continue QT awareness. |
Class adverse effects and interactions
- ECG risk: QT prolongation and torsades risk increase with high dose, structural heart disease, bradycardia, low potassium/magnesium, or combination with macrolides, fluoroquinolones, antipsychotics, methadone and other QT-prolonging drugs.
- Serotonin toxicity: uncommon but possible when combined with serotonergic medicines. Watch for agitation, hyperreflexia/clonus, fever and autonomic instability rather than assuming every tremor is “serotonin syndrome.”
- Constipation: important after surgery or in patients at risk of ileus; investigate obstruction rather than escalating anti-emetics.
- Critical interaction: ondansetron must not be combined with apomorphine because profound hypotension and loss of consciousness have been reported.
2. Dopamine D2-receptor antagonists and prokinetic drugs
D2 blockade in the CTZ reduces vomiting. Some agents also improve gastric emptying, which is helpful when nausea results from delayed gastric emptying. The trade-off is central dopamine blockade: acute dystonia, akathisia, parkinsonism, tardive dyskinesia and neuroleptic malignant syndrome are clinically important.
Metoclopramide
Metoclopramide blocks D2 receptors; at higher doses it also has 5-HT3-blocking activity. It enhances upper gastrointestinal motility by promoting acetylcholine release and has a useful prokinetic role in diabetic gastroparesis. It is not merely a “strong anti-emetic”; its mechanism helps determine when it is appropriate.
| Feature | Clinical teaching point |
|---|---|
| Common adult dose | 10 mg PO, IM or slow IV every 6–8 h as required. For diabetic gastroparesis, a commonly labelled regimen is 10 mg PO 30 minutes before meals and at bedtime for a limited course. Reduce dose in renal impairment. |
| Maximum/duration | Use the lowest effective dose and shortest duration. Avoid long-term use because tardive dyskinesia risk rises with cumulative exposure; many labels restrict therapy to ≤12 weeks except in unusual circumstances. |
| Do not use / use specialist caution | Mechanical bowel obstruction, perforation, GI bleeding, phaeochromocytoma, Parkinson disease, prior dystonic reaction/tardive dyskinesia, epilepsy or concurrent antipsychotic exposure. |
| Major adverse effects | Akathisia, acute dystonia (especially young adults), sedation, diarrhoea, hyperprolactinaemia, parkinsonism, tardive dyskinesia and rare neuroleptic malignant syndrome. |
Domperidone
Domperidone is a peripheral D2 antagonist with less penetration into the central nervous system than metoclopramide, therefore fewer extrapyramidal effects. Its major limitation is cardiac: QT prolongation and ventricular arrhythmia risk. It is not a casual long-term treatment for nonspecific dyspepsia.
- Common adult dose: 10 mg PO up to three times daily before meals; many safety recommendations limit total dose to 30 mg/day and advise short-term use.
- Avoid: known QT prolongation, significant cardiac disease, severe hepatic impairment, electrolyte disturbance, and combinations with QT-prolonging agents or potent CYP3A4 inhibitors.
- Do not assume “peripheral” means harmless: obtain an ECG/risk assessment where appropriate.
Phenothiazines, butyrophenones and other D2 blockers
| Drug | Common adult teaching dose | Best fit / major cautions |
|---|---|---|
| Prochlorperazine | 5–10 mg PO three times daily as needed, or 12.5 mg IM/IV in monitored clinical use; follow local product and route guidance. | Vertigo-associated vomiting, migraine-related nausea and selected acute nausea. Risk: dystonia, akathisia, sedation, hypotension, QT prolongation. Avoid in Parkinson disease. |
| Haloperidol | 0.5–1 mg PO/IM/IV in low-dose anti-emetic protocols; dose and IV use require local protocol/ECG monitoring. | Useful in selected palliative-care or refractory nausea pathways. EPS, QT prolongation, sedation and neuroleptic malignant syndrome are key hazards. |
| Droperidol | 0.625–1.25 mg IV/IM for PONV in monitored peri-operative settings, according to local protocol. | Effective but requires careful QT-risk assessment and monitoring policies. |
3. H1-antihistamines
These reduce vestibular and NTS signalling. Their strongest role is prevention or treatment of motion sickness and vestibular vertigo-associated nausea. Sedation and anticholinergic effects are common and may be dangerous in frail older adults, glaucoma, urinary retention or delirium.
| Drug | Common adult teaching dose | Key use and cautions |
|---|---|---|
| Promethazine | 12.5–25 mg PO/IM/IV every 4–6 h as required; for motion sickness 25 mg 30–60 min before travel then as directed by product protocol. | Antihistaminic, sedating, anticholinergic and anti-motion-sickness actions. Avoid in children <2 years due to fatal respiratory depression risk. IV administration can cause severe tissue injury if extravasated; use local safe-route guidance. |
| Cyclizine | 50 mg PO up to three times daily; 50 mg IM/slow IV up to three times daily where locally licensed. | Motion sickness, vestibular nausea and PONV. Causes drowsiness, dry mouth, blurred vision, urinary retention and confusion. |
| Dimenhydrinate | 50–100 mg PO/IM/IV every 4–6 h as required; follow product maximum daily dose. | Motion sickness and vestibular symptoms. Sedation/antimuscarinic toxicity; avoid combining with alcohol or other sedatives. |
| Meclizine | 25–50 mg PO 1 h before travel; repeat according to product guidance, commonly every 24 h for prevention. | Longer-acting motion-sickness prevention; sedating and anticholinergic. |
4. Antimuscarinic: hyoscine (scopolamine)
Hyoscine blocks muscarinic M1 signalling from the vestibular system to the vomiting centre. It is particularly effective for prevention of motion sickness, but it is not a good choice for established chemotherapy nausea or toxin-induced vomiting.
- Transdermal adult dose: apply one patch delivering approximately 1 mg over 3 days behind the ear at least 4 hours before motion-sickness prophylaxis is needed; do not cut the patch and do not wear more than one at a time.
- Adverse effects: dry mouth, blurred vision, mydriasis, constipation, urinary retention, drowsiness, confusion and rarely acute angle-closure glaucoma.
- Avoid/caution: angle-closure glaucoma, urinary retention/prostatic obstruction, severe cognitive impairment and concurrent anticholinergic medicines.
5. NK1-receptor antagonists
Substance P acts at NK1 receptors in the brainstem. NK1 antagonists are especially valuable for delayed CINV and are routinely combined with a 5-HT3 antagonist and dexamethasone for highly emetogenic chemotherapy. They are specialist/oncology-protocol drugs, not routine treatment for viral gastroenteritis.
| Drug | Example adult regimen | Important interactions |
|---|---|---|
| Aprepitant | 125 mg PO on day 1, then 80 mg PO on days 2 and 3 in a common 3-day CINV regimen; 40 mg PO once is used in some PONV protocols. | Moderate CYP3A4 inhibitor/inducer effects. Reduce dexamethasone dose in relevant CINV regimens; monitor warfarin INR and review hormonal contraceptive effectiveness. |
| Fosaprepitant | 150 mg IV once before chemotherapy in protocol-driven regimens. | Prodrug of aprepitant; same interaction principles. Check infusion-site reactions and product compatibility. |
6. Adjuvant anti-emetics
Dexamethasone
Corticosteroids enhance anti-emetic control in CINV and PONV through incompletely defined central and anti-inflammatory mechanisms. An oncology regimen must be protocol-based; a common high-emetic-risk CINV component is dexamethasone 12 mg PO/IV before chemotherapy, often with an NK1 antagonist, 5-HT3 antagonist and olanzapine. When aprepitant is used, dexamethasone doses are commonly reduced because of pharmacokinetic interaction.
Short courses can cause hyperglycaemia, insomnia, dyspepsia and mood change; infection risk and psychiatric effects matter with repeated or high-dose exposure.
Olanzapine
Olanzapine blocks several relevant receptors including D2, 5-HT2, H1 and muscarinic receptors. It is effective for prevention and rescue of CINV in specialist protocols.
- Common adult CINV dose: 5–10 mg PO once daily, often for 3–4 days as part of a protocol.
- Key limitation: sedation; also orthostatic hypotension, metabolic effects and anticholinergic burden. Use a lower dose or avoid where alertness is crucial.
Cannabinoids and benzodiazepines
Nabilone/dronabinol may be reserved for refractory CINV in settings where available; adverse effects include dysphoria, sedation, hallucinations and postural hypotension. Lorazepam is not a primary anti-emetic but may help anticipatory nausea driven by anxiety; it can cause respiratory depression when combined with opioids, alcohol or other sedatives.
Cause-directed prescribing
| Clinical scenario | Likely pathway | Reasonable drug direction | Do not miss |
|---|---|---|---|
| Motion sickness | Vestibular H1/M1 | Hyoscine patch, cyclizine, promethazine, meclizine as appropriate. | Glaucoma, urinary retention, sedation while travelling/driving. |
| Acute migraine with vomiting | Central/dopaminergic and visceral pathways | Metoclopramide or prochlorperazine in an emergency protocol, often with analgesia and fluids. | Subarachnoid haemorrhage, meningitis, raised intracranial pressure. |
| Gastroparesis | Delayed gastric emptying | Short-course metoclopramide after assessing obstruction and glycaemic control. | Mechanical obstruction; EPS risk and renal dose reduction. |
| PONV | Multifactorial | 5-HT3 antagonist, dexamethasone, droperidol/other class according to peri-operative protocol; use a different class for rescue if prophylaxis failed. | Airway, aspiration, postoperative bleeding/obstruction. |
| Highly emetogenic chemotherapy | 5-HT3, NK1, delayed pathways | Protocol-based 4-drug prevention: NK1 antagonist + 5-HT3 antagonist + dexamethasone + olanzapine. | Drug interactions, steroid-related hyperglycaemia, breakthrough vs delayed symptoms. |
| Pregnancy nausea/vomiting | Multifactorial | Use obstetric guidance; doxylamine/pyridoxine and selected antihistamines/phenothiazines are common first options. Metoclopramide, domperidone and ondansetron are generally later-line choices in guideline pathways. | Ectopic pregnancy, molar pregnancy, hepatitis, UTI, thyrotoxicosis, severe dehydration and hyperemesis gravidarum. |
Pregnancy and hyperemesis gravidarum: safe thinking
Assess gestation, hydration, ketones, weight loss, electrolytes, abdominal pain, fever, urinary symptoms, bleeding and alternate diagnoses. Give thiamine before glucose-containing fluids in prolonged vomiting or malnutrition risk. Drug selection and fetal-safety counselling must follow local obstetric guidance; route may need to be parenteral or rectal when oral medicines cannot be retained. Avoid indiscriminate use of multiple QT-prolonging agents in dehydrated patients.
Emergency assessment checklist
- ABCDE: protect airway in active vomiting or impaired consciousness; monitor oxygenation and circulation.
- Red flags: bilious/feculent vomiting, haematemesis, rigid abdomen, severe headache/neurology, meningism, chest pain, severe dehydration, oliguria, hypotension, fever, poisoning or altered mental state.
- Targeted tests: glucose, pregnancy test when applicable, electrolytes/renal function, ECG when QT-risk medicines are used, and cause-directed imaging/laboratory studies.
- Fluid/electrolyte correction: anti-emetics do not correct hypovolaemia, hypokalaemia, hypochloraemia or acidosis/alkalosis.
- Review the medicine list: opioids, digoxin, antibiotics, antiretrovirals, chemotherapeutic agents, antiepileptics and antidepressants may cause nausea or interact with anti-emetics.
Drug-interaction and prescribing traps
- QT stacking: do not casually combine ondansetron, domperidone, prochlorperazine/haloperidol and other QT-prolonging drugs without risk assessment and ECG/electrolyte review.
- Extrapyramidal stacking: avoid combining metoclopramide with antipsychotics when possible; monitor for dystonia and tardive dyskinesia.
- Sedation stacking: promethazine, cyclizine, benzodiazepines, opioids and alcohol can combine to depress alertness and respiration.
- Anticholinergic burden: hyoscine, promethazine and cyclizine can precipitate delirium, urinary retention, constipation and blurred vision.
- Parkinson disease: dopamine antagonists can markedly worsen motor symptoms; seek an alternative plan.
- Paediatrics: never copy adult doses. Age/weight restrictions and risks, especially respiratory depression and dystonia, differ by product.
Clinical cases
Case 1: vomiting after cisplatin
A patient receives highly emetogenic chemotherapy. Ondansetron alone may control some acute symptoms but does not adequately prevent delayed emesis. The preventive plan should follow oncology protocol and combine a 5-HT3 antagonist, NK1 antagonist, dexamethasone and olanzapine, while checking interaction-driven steroid dose adjustment.
Case 2: young adult develops neck spasm after metoclopramide
Acute painful torticollis and upward eye deviation shortly after a D2 antagonist are an acute dystonic reaction until proven otherwise. Stop the causative medicine, assess airway and treat urgently under local protocol. Document the reaction to prevent repeat exposure.
Case 3: elderly patient with vomiting, potassium 2.8 mmol/L and prolonged QT
Do not add multiple QT-prolonging anti-emetics. Correct electrolytes, obtain ECG monitoring, investigate the cause of vomiting and select an option only after risk–benefit assessment.
High-yield revision points
- 5-HT3 antagonists are strong agents for CINV/PONV, not motion sickness.
- H1 antihistamines and hyoscine are the classic vestibular/motion-sickness drugs.
- Metoclopramide is both a D2 antagonist and prokinetic; obstruction and EPS history matter.
- Domperidone has fewer CNS EPS effects but can cause serious QT-related arrhythmia.
- Highly emetogenic chemotherapy often needs four prophylactic classes, not one “rescue” medicine.
- Antiemetic prescribing is unsafe when the cause of vomiting and the patient’s airway, hydration and ECG risk have not been assessed.
