Anti-Emetics in Dogs: Decision Guide for Clinicians — GlobalVetCo

Anti-Emetics in Dogs: Decision Guide for Clinicians

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Receptor pharmacology (NK1, 5-HT3, D2, H1), head-to-head drug comparisons, a case-based antiemetic selection algorithm, and condition-specific protocols for chemotherapy-induced nausea, motion sickness, pancreatitis, and parvoviral enteritis — the complete emesis management guide for dogs.

Key takeaways
  • Maropitant (Cerenia) is the most broad-spectrum antiemetic available for dogs — it blocks the NK1 receptor in the emetic centre and is effective against peripheral, central, and chemoreceptor-trigger-zone stimuli.
  • Ondansetron is a 5-HT3 antagonist with particular efficacy against chemotherapy-induced nausea — but it has no effect on motion sickness and minimal effect on GI inflammation-induced vomiting.
  • Metoclopramide is primarily a prokinetic with mild antiemetic effects at high doses — it should NOT be used as a sole antiemetic for moderate-to-severe vomiting.
  • Multimodal antiemetic therapy (maropitant + ondansetron) addresses multiple receptor pathways simultaneously and is superior to monotherapy for severe or refractory vomiting.
  • Anti-emetics do not treat the underlying disease — they are supportive therapy. Always investigate the cause of vomiting in parallel with symptomatic management.
Red flags / do not miss
  • Vomiting with haematemesis (frank blood or coffee-ground material) suggests GI ulceration or coagulopathy — do not simply prescribe an antiemetic. Investigate with PCV, coagulation profile, and abdominal imaging.
  • Projectile vomiting in a young dog, especially after eating, suggests a vascular ring anomaly (persistent right aortic arch — PRAA) or pyloric stenosis. An antiemetic may mask the diagnosis — obtain thoracic and abdominal radiographs first.
  • Bilious vomiting syndrome (early-morning vomiting of bile) is caused by duodenogastric reflux of bile in the fasted state — it responds to a late-night small meal or a prokinetic (metoclopramide), not to an antiemetic.
  • An antiemetic should never be used to 'settle the stomach' so a vomiting dog can be sent home without a diagnosis — vomiting that is severe enough to require an antiemetic is severe enough to require investigation.
  • High-dose metoclopramide (CRI > 2 mg/kg/day) can cause extrapyramidal signs (tremors, rigidity, agitation) — this is a D2-antagonist effect identical to the neuroleptic malignant syndrome seen with antipsychotics in humans.

1. Introduction: The Physiology of Vomiting

Vomiting is a complex, centrally coordinated reflex that evolved to protect the body from ingested toxins. It involves the integration of signals from four anatomically and pharmacologically distinct input pathways, all converging on the emetic centre in the medulla oblongata. Understanding which pathway is activated by a given clinical condition allows rational antiemetic drug selection — instead of the 'try maropitant and see' approach that dominates current practice.

This guide covers the receptor pharmacology of the four major antiemetic drug classes (NK1 antagonists, 5-HT3 antagonists, D2 antagonists, and H1 antagonists), provides head-to-head comparisons of maropitant, ondansetron, and metoclopramide, and presents a case-based algorithm for antiemetic selection across the most common clinical scenarios.

2. The Emetic Pathways: Four Routes to the Emetic Centre

Input Pathway Stimulus Detected By Key Receptors Emetic Triggers Most Effective Drug Class
Chemoreceptor trigger zone (CRTZ) Area postrema (floor of 4th ventricle — OUTSIDE the blood–brain barrier) NK1, D2, 5-HT3 Uraemic toxins, ketones (DKA), hypercalcaemia, chemotherapy drugs (cisplatin, doxorubicin), apomorphine, cardiac glycosides Maropitant (NK1) + Ondansetron (5-HT3) for chemotherapy
Vestibular system Semicircular canals and vestibular nuclei H1, M1 (muscarinic) Motion sickness, vestibular disease (idiopathic vestibular syndrome, otitis media/interna) Maropitant (NK1) — also effective. Meclizine or dimenhydrinate (H1) are alternatives.
Gastrointestinal tract (peripheral) Enterochromaffin cells in the GI mucosa release serotonin (5-HT) when irritated 5-HT3 (vagal afferents) Dietary indiscretion, gastroenteritis, pancreatitis, parvoviral enteritis, gastric foreign body, inflammatory bowel disease Maropitant (NK1) — superior to ondansetron for GI inflammation. Ondansetron (5-HT3) is second-line.
Cerebral cortex / limbic system Psychological stimuli (fear, anxiety, learned aversion) NK1, H1, GABA, opioid receptors Anticipatory vomiting (chemotherapy), severe pain, raised intracranial pressure, anxiety Maropitant (NK1). Benzodiazepines for anticipatory nausea.
Key concept
The emetic centre is the final common pathway. Maropitant blocks the NK1 receptor in the emetic centre itself — this is why it works for ALL types of vomiting, regardless of the initiating stimulus. Other antiemetics block only specific input pathways.

3. Receptor Pharmacology of the Four Major Antiemetic Classes

3.1 NK1 Receptor Antagonists — Maropitant (Cerenia)

Substance P is the endogenous ligand for the neurokinin-1 (NK1) receptor. NK1 receptors are abundant in the emetic centre, the CRTZ, and the GI tract. Maropitant is a selective, high-affinity NK1 antagonist that blocks substance-P-mediated emetic signalling at all three sites. It is effective against peripheral, central, and CRTZ-mediated vomiting.

  • Onset: 1–2 hours PO, < 30 minutes SC/IV.
  • Duration: 24 hours (once-daily dosing).
  • Additional effects: Visceral analgesic (blocks NK1 receptors in the spinal cord dorsal horn). Anxiolytic at high doses (central NK1 receptors modulate anxiety).
  • Adverse effects: Pain on SC injection (formulated at low pH — buffer with sterile water or sodium bicarbonate if injecting undiluted). Diarrhoea (rare). Bone-marrow suppression (reported at very high, prolonged doses — not at clinical doses).

3.2 5-HT3 Receptor Antagonists — Ondansetron, Granisetron, Dolasetron

Serotonin (5-hydroxytryptamine, 5-HT) released from enterochromaffin cells in response to GI mucosal irritation (chemotherapy, radiation, inflammation) activates 5-HT3 receptors on vagal afferent nerve terminals, triggering the emetic reflex. 5-HT3 antagonists block these peripheral vagal signals as well as 5-HT3 receptors in the CRTZ.

  • Ondansetron: 0.5–1 mg/kg IV/PO q8h (short half-life: ~1.5 hours in dogs). Most commonly used 5-HT3 antagonist.
  • Granisetron: 0.01–0.02 mg/kg IV q12–24h (longer half-life). Superior palatability in cats (transdermal formulation available).
  • Dolasetron: 0.6–1 mg/kg IV/PO q24h. Prodrug — converted to hydrodolasetron, which has a longer half-life than ondansetron.
  • Key limitation: 5-HT3 antagonists have NO efficacy against motion sickness and limited efficacy against CRTZ-mediated vomiting not involving serotonin (uraemia, DKA). They are most effective for chemotherapy-induced and GI-inflammatory vomiting.

3.3 D2 Dopamine Receptor Antagonists — Metoclopramide

Metoclopramide blocks D2 dopamine receptors in the CRTZ, providing a mild central antiemetic effect. It is also a 5-HT4 agonist and a weak 5-HT3 antagonist at high doses, which accounts for its prokinetic effects (enhanced gastric emptying, increased lower oesophageal sphincter tone).

  • Dose: 0.2–0.5 mg/kg PO/SC/IM q8h, or 1–2 mg/kg/day as a continuous IV infusion.
  • Antiemetic efficacy: Weak to moderate at standard doses. Not adequate as a sole antiemetic for moderate-to-severe vomiting.
  • Prokinetic efficacy: Good — enhances gastric emptying. Useful for gastroparesis and gastro-oesophageal reflux.
  • Adverse effects: Extrapyramidal signs (tremors, rigidity, agitation) at high doses — reversible with diphenhydramine. Contraindicated in GI obstruction (promoting motility against an obstruction risks perforation).

3.4 H1 Histamine Receptor Antagonists — Dimenhydrinate, Meclizine, Diphenhydramine

H1 antagonists block histamine-mediated signalling from the vestibular system to the emetic centre. They are primarily indicated for motion sickness and vestibular disease. They have minimal efficacy against GI or CRTZ-mediated vomiting and should not be used as sole antiemetics for these conditions.

4. Head-to-Head Drug Comparisons

Parameter Maropitant (Cerenia) Ondansetron Metoclopramide Meclizine / Dimenhydrinate
Receptor target NK1 5-HT3 D2 (CRTZ); 5-HT4 agonist (GI); weak 5-HT3 H1 (vestibular), M1 (muscarinic)
Efficacy — motion sickness HIGH (FDA-approved) NONE LOW MODERATE–HIGH
Efficacy — chemotherapy vomiting HIGH HIGH (gold standard in human oncology) LOW–MODERATE NONE
Efficacy — GI inflammation (pancreatitis, gastroenteritis, parvo) HIGH MODERATE LOW–MODERATE (prokinetic effect helps gastric emptying) NONE
Efficacy — uraemia/DKA vomiting HIGH MODERATE MODERATE NONE
Prokinetic effect NONE NONE GOOD NONE
Analgesic effect YES (visceral) NONE NONE NONE
Dosing frequency q24h q8h (short half-life) q8h PO/SC/IM; CRI q8–12h (meclizine q24h)
Route SC, IV, PO IV, PO SC, IM, IV, PO, CRI PO only (meclizine)
Cost (relative) $$$ $$ $ $
Veterinary licensing Licensed for dogs (vomiting, motion sickness) Off-label (human drug) Licensed for dogs Off-label (human drug)

5. Case-Based Antiemetic Selection Algorithm

5.1 Scenario 1: Dietary Indiscretion / Mild Gastroenteritis

A 2-year-old dog presents with acute vomiting of 12 hours' duration after raiding the bin. Alert, hydrated, no abdominal pain.

  • Recommendation: Maropitant 2 mg/kg PO q24h for 2–3 days, or 1 mg/kg SC once followed by PO at home. Withhold food for 12 hours, then introduce a bland diet (boiled chicken and rice) in small, frequent meals.
  • Rationale: Maropitant blocks both peripheral GI and central pathways. Single daily dosing is practical for owners. Metoclopramide alone is insufficient; ondansetron (q8h) is less practical for at-home therapy.

5.2 Scenario 2: Moderate–Severe Pancreatitis

A 6-year-old Miniature Schnauzer presents with acute vomiting, abdominal pain, lethargy, and a cPLI > 400 μg/L. Hospitalised on IV fluids.

  • Recommendation: Maropitant 1 mg/kg IV q24h. Add ondansetron 0.5 mg/kg IV q8h if vomiting persists despite maropitant. Metoclopramide CRI (1–2 mg/kg/day) if ileus is present (confirmed by ultrasound showing reduced GI motility).
  • Rationale: Pancreatitis triggers vomiting via GI inflammation (5-HT release) and systemic inflammation (CRTZ stimulation). Maropitant covers both pathways and provides additional visceral analgesia. Ondansetron is a rational add-on if monotherapy is insufficient. Metoclopramide is for ileus management, not primarily for vomiting.

5.3 Scenario 3: Chemotherapy-Induced Nausea and Vomiting

A dog receiving doxorubicin (which is highly emetogenic) for lymphoma. Pre-emptive antiemesis is planned.

  • Recommendation: Maropitant 1 mg/kg SC 30–60 minutes before chemotherapy, then 2 mg/kg PO q24h for 3–5 days. Add ondansetron 0.5 mg/kg IV 30 minutes before chemotherapy if the dog has previously experienced vomiting despite maropitant alone.
  • Rationale: Chemotherapy drugs (especially cisplatin and doxorubicin) release serotonin from enterochromaffin cells, which activates 5-HT3 receptors on vagal afferents. Ondansetron is the gold standard in human oncology for this pathway. However, maropitant (which blocks the emetic centre directly) is also effective, and its once-daily dosing and visceral-analgesic effects make it a convenient first choice in dogs. For highly emetogenic protocols, dual therapy (maropitant + ondansetron) is superior to either drug alone.

5.4 Scenario 4: Parvoviral Enteritis

A 12-week-old unvaccinated puppy with haemorrhagic diarrhoea, severe vomiting, and neutropenia. Confirmed CPV-positive on SNAP test.

  • Recommendation: Maropitant 1 mg/kg IV q24h. Add ondansetron 0.5 mg/kg IV q8h if vomiting is severe and interfering with oral intake. Metoclopramide is generally not indicated — the intensive GI inflammation is the cause, not gastric stasis.
  • Rationale: CPV causes intense GI mucosal inflammation with massive serotonin release. Maropitant is first-line for broad-spectrum coverage. Ondansetron provides complementary 5-HT3 blockade. The combination is synergistic.

5.5 Scenario 5: Motion Sickness (Prophylaxis)

A 3-year-old dog with a history of profuse salivation and vomiting within 20 minutes of starting a car journey.

  • Recommendation: Maropitant 8 mg/kg PO (the higher, motion-sickness dose) 2 hours before travel. Alternatively, meclizine 4 mg/kg PO q24h starting the night before travel.
  • Rationale: Maropitant is FDA-approved for canine motion sickness at the 8 mg/kg dose. It blocks NK1 receptors in the emetic centre, which receives vestibular input. Meclizine (H1 antagonist) is a less expensive but less well-studied alternative. Ondansetron and metoclopramide are not effective for motion sickness.

5.6 Scenario 6: Uraemic Vomiting (Chronic Kidney Disease)

A 14-year-old cat with Stage 3 CKD, chronic intermittent vomiting, and a BUN > 80 mg/dL.

  • Recommendation: Maropitant 1 mg/kg PO q24h (dog dose; in cats, 1 mg/kg PO q24h is also used). Metoclopramide 0.2–0.5 mg/kg PO q8h may be added for its prokinetic effect if gastric emptying is delayed.
  • Rationale: Uraemic toxins stimulate the CRTZ (outside the BBB) via NK1 and D2 receptors. Maropitant blocks the NK1 pathway. Metoclopramide provides D2 blockade in the CRTZ and a prokinetic effect. The combination addresses multiple pathways simultaneously.

6. Practical Prescribing Guide

Drug Dose (Dog) Route Frequency Indications Cost Category
Maropitant (Cerenia) 1 mg/kg (antiemetic); 2 mg/kg PO (severe); 8 mg/kg PO (motion sickness prophylaxis) SC, IV, PO q24h First-line for all vomiting types. Prophylaxis for motion sickness. $$$
Ondansetron 0.5–1 mg/kg IV (slow), PO q8h Chemotherapy vomiting. Second-line add-on for pancreatitis, parvo. $$
Metoclopramide 0.2–0.5 mg/kg SC, IM, IV, PO q8h Prokinetic for gastroparesis, reflux. Mild antiemetic — not for sole therapy of moderate–severe vomiting. $
Metoclopramide CRI 1–2 mg/kg/day IV CRI Continuous Ileus, severe gastroparesis. Hospitalised patients only. $
Meclizine 4 mg/kg (dog); 2–4 mg/kg (cat) PO q24h (dog) Motion sickness (alternative to maropitant). Vestibular disease. $
Dimenhydrinate (Dramamine) 4–8 mg/kg PO q8h Motion sickness. Less preferred than maropitant or meclizine. $
Diphenhydramine (Benadryl) 2–4 mg/kg PO, IM q8h Extrapyramidal signs from metoclopramide overdose. Vestibular nausea (minor role). $
Multimodal antiemesis rule
If vomiting is severe or refractory to a single agent, add a drug from a DIFFERENT receptor class — not a different drug from the same class. Maropitant (NK1) + ondansetron (5-HT3) is the most effective combination and covers the broadest spectrum of emetic pathways.

7. Clinical Decision Algorithm

Follow this algorithm for any vomiting dog:

  1. Step 1 — Assess severity and stability: Is the patient haemodynamically stable? If hypotensive, tachycardic, or collapsed → IV fluids, investigate cause of shock (haemorrhagic gastroenteritis, GDV, pancreatitis), and start antiemetic therapy once stabilised.
  2. Step 2 — Identify the likely emetic pathway: GI inflammation? CRTZ stimulation (uraemia, DKA, drugs)? Vestibular? Motion-related? Use the table in Section 2 to identify the dominant pathway.
  3. Step 3 — Select primary antiemetic: Maropitant is the default first choice for all pathways. Modify if cost prohibits maropitant: ondansetron for chemotherapy/GI; metoclopramide only for mild vomiting or prokinetic need; meclizine for motion sickness.
  4. Step 4 — Assess response at 24 hours: Is vomiting controlled? If yes → continue current therapy. If no → add a second agent from a different receptor class (ondansetron is the most common add-on).
  5. Step 5 — Investigate the underlying cause: Anti-emetics are a bridge, not a destination. Perform abdominal imaging (radiographs ± ultrasound), CBC, biochemistry, and specific diagnostics (cPLI, cortisol, ACTH stim, parvovirus test) as indicated by the clinical picture.
Clinical pearls
  • The CRTZ (chemoreceptor trigger zone) is outside the blood–brain barrier — it 'samples' the blood for circulating emetogenic substances. This is why uraemic toxins, ketones, and chemotherapy drugs trigger vomiting even without GI pathology.
  • Maropitant is also an analgesic — NK1 receptors are involved in pain transmission in the spinal cord. A single dose of maropitant (1 mg/kg SC) provides visceral analgesia comparable to buprenorphine in some models.
  • Ondansetron has a very short half-life in dogs (~1.5 hours) — for sustained antiemesis, it should be dosed q8h (not q12h or q24h as sometimes prescribed).
  • Metoclopramide crosses the blood–brain barrier and blocks D2 receptors in the CRTZ, but at standard doses (0.2–0.5 mg/kg) its antiemetic effect is weak. The prokinetic effect (enhanced gastric emptying) may be its primary contribution to vomiting control.
  • Famotidine or omeprazole are not antiemetics — they reduce gastric acid but do not block any emetic pathway. They may be adjunctive in gastritis-associated vomiting but should not replace a true antiemetic.

Frequently asked questions

Which antiemetic works fastest?
Maropitant administered IV or SC has an onset of action within 30 minutes. IV ondansetron works in 15–30 minutes. For the fastest effect in an actively vomiting patient, IV maropitant (1 mg/kg over 1–2 minutes) is the standard in emergency practice.
Can I combine maropitant and metoclopramide?
Yes — this is a rational combination because the drugs target different receptors (NK1 vs D2/5-HT4) and have complementary effects (antiemetic vs prokinetic). This combination is particularly useful in pancreatitis, where vomiting and ileus frequently coexist.
Why don't we use metoclopramide as a sole antiemetic anymore?
Metoclopramide's antiemetic effect at standard doses (0.2–0.5 mg/kg) is weak because its D2 receptor blockade in the CRTZ addresses only one of several emetic pathways. Maropitant and ondansetron are substantially more effective. Metoclopramide still has an important role as a prokinetic — it promotes gastric emptying and increases lower oesophageal sphincter tone — but prescribing it as a sole antiemetic for a vomiting patient is likely to result in treatment failure.
Is famotidine an antiemetic?
No. Famotidine is an H2 receptor antagonist that reduces gastric acid secretion. It does not block any emetic pathway. It may be a useful adjunct when gastric hyperacidity contributes to nausea (uraemic gastritis, mast cell tumour), but it should never be relied upon to control vomiting.
Is maropitant safe in puppies?
Maropitant is licensed for use in dogs from 8 weeks of age. Studies in puppies as young as 7 weeks have demonstrated safety. Dose reduction may be appropriate in very small puppies (< 2 kg) due to the difficulty of accurate dosing with the injectable formulation. Bone marrow suppression has been reported in puppies receiving maropitant at doses higher than recommended for prolonged periods — use the minimum effective dose.
How long should I continue antiemetics after vomiting stops?
For acute vomiting (dietary indiscretion, mild gastroenteritis): continue for 24–48 hours after the last vomiting episode, then discontinue. For chronic conditions (pancreatitis, CKD, chemotherapy): continue for the duration of the emetogenic stimulus, tapering as the stimulus resolves. There is no evidence for a physiological 'rebound' vomiting upon antiemetic discontinuation.
What is the best antiemetic for a dog with bilious vomiting syndrome?
Neither — bilious vomiting syndrome (BVS) is caused by duodenogastric reflux of bile during prolonged fasting, not by a true emetic stimulus. The treatment is to feed a small, late-night meal to reduce the fasting interval, and to use a prokinetic (metoclopramide 0.2–0.5 mg/kg PO at bedtime) to promote gastric emptying. If these measures fail, maropitant can be tried, but addressing the underlying reflux is more effective than blocking the emetic centre.
What antiemetic should I use for a vomiting cat?
Maropitant at 1 mg/kg SC/PO q24h is the first-line antiemetic in cats and is licensed for feline use in many countries. Ondansetron (0.5 mg/kg IV/PO q8–12h) is the most common second-line agent. Metoclopramide (0.2–0.5 mg/kg PO/SC q8h) is used as a prokinetic. Cats with chronic kidney disease and uraemic vomiting often benefit from the combination of maropitant + metoclopramide (different receptor targets).

Self-check quiz

Test yourself. Answers are below each question — cover them first if you are studying.

Q1. A dog receiving cisplatin chemotherapy develops severe vomiting 4 hours after treatment despite receiving maropitant 1 hour before. What is the most appropriate next step?
  1. Increase maropitant to 2 mg/kg IV
  2. Add ondansetron 0.5 mg/kg IV q8h
  3. Switch maropitant to metoclopramide CRI
  4. Add famotidine 0.5 mg/kg IV
  5. Discontinue chemotherapy — the dog cannot tolerate it
Show answer

Answer: Add ondansetron 0.5 mg/kg IV q8h

Cisplatin is highly emetogenic and releases massive amounts of serotonin from enterochromaffin cells, activating 5-HT3 receptors. Maropitant blocks NK1 receptors in the emetic centre — effective, but when monotherapy fails, adding ondansetron (5-HT3 antagonist) targets the missing pathway. Dual NK1 + 5-HT3 blockade is the standard of care in human oncology for highly emetogenic chemotherapy.

Q2. Which of the following antiemetics is LEAST likely to be effective for vomiting caused by pancreatitis in a dog?
  1. Maropitant
  2. Ondansetron
  3. Metoclopramide (prokinetic)
  4. Meclizine (H1 antagonist)
  5. Maropitant + ondansetron combination
Show answer

Answer: Meclizine (H1 antagonist)

Meclizine (and other H1 antagonists) primarily addresses vestibular (motion-sickness) input to the emetic centre. Pancreatitis triggers vomiting via GI inflammation (5-HT release → vagal stimulation) and systemic inflammation (CRTZ). H1 blockade does not interrupt these pathways. Maropitant is the first-line choice, with ondansetron as the most rational add-on.

Q3. What is the primary advantage of maropitant over ondansetron for at-home antiemetic therapy?
  1. Maropitant is more effective against chemotherapy vomiting
  2. Maropitant is less expensive
  3. Maropitant is dosed once daily (q24h) vs ondansetron q8h
  4. Maropitant has a faster onset of action
  5. Maropitant is available over-the-counter
Show answer

Answer: Maropitant is dosed once daily (q24h) vs ondansetron q8h

Ondansetron has a very short half-life in dogs (~1.5 hours) and requires q8h dosing for sustained effect — a significant practical burden for owners. Maropitant's 24-hour duration makes it far more convenient for at-home therapy. Both are similarly effective for most vomiting types, though ondansetron may be superior for purely serotonin-mediated (chemotherapy) vomiting.

Q4. A 3-month-old puppy with parvoviral enteritis is vomiting 8–10 times per day despite receiving maropitant 1 mg/kg IV q24h and IV fluids. The puppy is stable but cannot retain oral medications. What is the best intervention?
  1. Increase maropitant to 2 mg/kg IV q12h
  2. Add ondansetron 0.5 mg/kg IV q8h
  3. Add metoclopramide CRI
  4. Place a nasogastric tube and give maropitant enterally
  5. Accept the vomiting — it will resolve as the virus runs its course
Show answer

Answer: Add ondansetron 0.5 mg/kg IV q8h

When maropitant alone is insufficient for severe, CPV-associated vomiting, add ondansetron. The two drugs target different receptors (NK1 vs 5-HT3) and the combination covers the broadest spectrum of emetic pathways. Maropitant should not be dosed q12h (its duration is 24 hours). A nasogastric tube is contraindicated in a vomiting patient (risk of reflux and aspiration). Vomiting 8–10×/day is clinically significant and must be controlled.

Q5. Which emetic pathway does the chemoreceptor trigger zone (CRTZ) primarily utilise?
  1. The CRTZ does not exist in dogs
  2. It directly innervates the stomach to induce vomiting
  3. It samples blood for emetogenic substances and signals the emetic centre via NK1, D2, and 5-HT3 receptors
  4. It receives signals from the vestibular system only
  5. It is blocked exclusively by metoclopramide
Show answer

Answer: It samples blood for emetogenic substances and signals the emetic centre via NK1, D2, and 5-HT3 receptors

The area postrema (CRTZ) is unique in that it lies outside the blood–brain barrier, allowing it to 'sample' the blood for circulating emetogenic substances (uraemic toxins, ketones, chemotherapy drugs). It signals the emetic centre through multiple receptor types — NK1 (substance P), D2 (dopamine), and 5-HT3 (serotonin) — which is why multi-receptor antiemetic therapy is sometimes required.

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Educational disclaimer: This article is for veterinary students and licensed professionals. It is not a substitute for case-specific clinical judgment, local formulary rules, or current drug labels. Always verify doses, legality, and species contraindications before treating.
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