Ondansetron
Ondansetron is a selective serotonin 5-HT3-receptor antagonist used for nausea and vomiting in veterinary patients. Important applications include emetogenic chemotherapy and selected gastrointestinal or systemic diseases. Its receptor profile provides a complementary approach to antiemetics acting through dopamine or neurokinin pathways, although evidence varies among individual clinical indications.
Group
5-HT3 antagonist antiemetic
Mechanism of Action
Ondansetron blocks 5-HT3 receptors on gastrointestinal vagal afferents and in central emetic pathways. Serotonin released from enterochromaffin cells after gastrointestinal stimulation or injury can activate these receptors and initiate emetic signaling. Receptor antagonism interrupts that input and reduces activation of the vomiting response. Central receptor effects contribute to activity against additional emetic stimuli.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Chemotherapy-associated vomiting | 0.5–1 mg/kg | IV, SC | Once | Give 30 minutes before IV chemotherapy. |
| Vomiting | 0.1–0.2 mg/kg | IV | Every 6–12 h | Inject slowly; increase to 0.5 mg/kg if initially ineffective. |
| Vomiting | 0.5 mg/kg | IV | Every 6–12 h | Escalation option after inadequate initial response. |
| Uremia-associated vomiting | 0.6–1 mg/kg | PO, IV | Every 12 h | Dogs: reported with metoclopramide; feline use is instead described under pancreatitis, so use condition-specific selection. |
| Vomiting | 0.1–0.3 mg/kg | IV | Every 8–12 h | Slow injection. |
| Vomiting | 0.1–1 mg/kg | PO | Every 12–24 h | |
| Chemotherapy-associated vomiting | 0.5–1 mg/kg | PO | Every 8–12 h | |
| Chemotherapy-associated vomiting | 0.1–0.5 mg/kg | IV | Every 8 h | Over 15 minutes, or begin 30 minutes before cisplatin. |
| Acute diarrhea adjunct | 0.5–1 mg/kg | PO | Every 12 h |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Vomiting | 0.5 mg/kg | SC, IV, PO | Every 8 h | |
| Vomiting | 2 mg/animal | PO | Every 8 h | Common fixed oral alternative. |
| Vomiting with hepatic disease | 0.5 mg/kg | SC, IV, PO | Every 12 h | |
| Antiemetic infusion — Loading | 0.5 mg/kg | IV | Once | |
| Antiemetic infusion — Maintenance | 0.5 mg/kg/hour | IV | Continuous | |
| Vomiting | 0.5 mg/kg | IV, PO | Every 12 h | |
| Intractable vomiting | 0.1–0.15 mg/kg | IV | Every 6–12 h | Slow push as needed. |
| Pancreatitis-associated vomiting | 0.1–1 mg/kg | PO, IV | Every 12–24 h |
Indications
- Prevention or treatment of chemotherapy-associated nausea and vomiting in dogs and cats.
- Treatment of selected gastrointestinal disease-associated nausea or vomiting, including some pancreatitis cases.
- Symptomatic treatment of selected systemic disease-associated nausea or vomiting, including some uremic patients.
- Prevention or reduction of selected sedative- or anesthetic-associated emesis, particularly in cats.
Contraindications
- Feline hepatic disease reduces clearance; decrease frequency.
- Transdermal application is ineffective in cats; oral bioavailability is low in dogs.
Interactions
- May precipitate if mixed with other IV drugs, including metoclopramide.
- Other QT-prolonging medicines may increase arrhythmia risk. Severe hypotension with apomorphine is reported in humans; veterinary relevance requires caution rather than a presumed safe combination.
Toxicity
Veterinary overdose information is limited; monitor cardiovascular and clinical effects and treat supportively.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Ondansetron 4 mg | Tablet | 4 mg / tablet | PO |
| Ondansetron 8 mg | Tablet | 8 mg / tablet | PO |
| Ondansetron syrup 4 mg/5 mL | Oral Liquid | 4 mg / 5 mL | PO |
| Ondansetron injection 2 mg/mL | Injection | 2 mg / mL | IV, SC |
Monitoring and Administration
Assess nausea, vomiting and hydration. Oral efficacy in dogs is uncertain because exposure is low and brief.
Keep solutions below pH 6 to prevent precipitation. Reported acidic oral vehicles retained stability for 42 days.
The additional oral canine regimens are reported clinical alternatives; low oral bioavailability remains a limitation and was not erased by adding them.