Ondansetron Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.5-1 mg/kg	q12h or q24h	Duration: As needed, typically 2-5 days Notes: For chemotherapy-induced emesis, administer 30 minutes before chemotherapy. For acute vomiting, may use q8h in severe cases.
Dog	IV	0.5-1 mg/kg (slow IV bolus)	q12h or q24h	Duration: As needed Notes: Administer slowly over 2-5 minutes to avoid hypotension. For severe vomiting or when oral route is not possible.
Cat	PO	0.5-1 mg/kg	q12h or q24h	Duration: As needed, typically 2-5 days Notes: Oral bioavailability is lower in cats; consider higher end of dose range. May be used for chemotherapy-induced emesis.
Cat	IV	0.5-1 mg/kg (slow IV bolus)	q12h or q24h	Duration: As needed Notes: Administer slowly. For severe vomiting or when oral route is not possible.
Horse	IV	0.1-0.2 mg/kg	q8h or q12h	Duration: As needed Notes: Limited data; use with caution. May be used for nausea associated with colic or post-operative.
Cattle	IV	0.1-0.2 mg/kg	q12h	Duration: As needed Notes: Not commonly used; ruminants rarely vomit. Use only if indicated.
Small Ruminants (Sheep/Goats)	IV	0.1-0.2 mg/kg	q12h	Duration: As needed Notes: Not commonly used; use with caution.
Rabbit	PO	0.5-1 mg/kg	q12h	Duration: As needed Notes: Rabbits cannot vomit, but may be used for nausea-related anorexia. Limited evidence.
Bird (Psittacines)	PO	0.5-1 mg/kg	q12h	Duration: As needed Notes: Limited data; use with caution.
Exotic (Ferrets)	PO	0.5-1 mg/kg	q12h	Duration: As needed Notes: Limited data; use with caution.

Clinical Indications & Species Uses

General Indications

Antiemetic for chemotherapy-induced emesis
Postoperative nausea and vomiting
Symptomatic treatment of vomiting due to various causes

Dog (Canine)

Prevention and treatment of nausea and vomiting associated with chemotherapy
Postoperative nausea and vomiting
Vomiting due to gastroenteritis, pancreatitis, renal disease, or vestibular disease
Motion sickness (off-label)

Cat (Feline)

Prevention and treatment of nausea and vomiting associated with chemotherapy
Postoperative nausea and vomiting
Vomiting due to gastrointestinal disease, pancreatitis, or renal disease

Horse (Equine)

Treatment of nausea and vomiting (rare in horses; used for colic-associated nausea or post-operative vomiting)

Pharmacology & Mechanism of Action

Drug Class: Antiemetic | Pharmacological Group: 5-HT3 receptor antagonist

Mechanism of Action: Ondansetron is a selective 5-HT3 receptor antagonist. It blocks the action of serotonin (5-hydroxytryptamine) at 5-HT3 receptors located both peripherally on vagal nerve terminals and centrally in the chemoreceptor trigger zone (CTZ) of the area postrema. By antagonizing these receptors, ondansetron inhibits the vomiting reflex triggered by chemotherapy, radiation, toxins, or other stimuli that release serotonin from enterochromaffin cells in the gastrointestinal tract.

Pharmacodynamics: Ondansetron has no effect on dopamine receptors, muscarinic receptors, or GABA receptors, which distinguishes it from other antiemetics. It effectively prevents nausea and vomiting induced by cisplatin and other emetogenic chemotherapeutic agents, as well as postoperative nausea and vomiting. In veterinary patients, it is used to manage vomiting associated with gastroenteritis, pancreatitis, renal disease, and chemotherapy. Its antiemetic effect is dose-dependent and more pronounced against serotonin-mediated emesis.

⚡ Pharmacokinetics Summary

Absorption: In dogs, ondansetron is well absorbed after oral administration, with a bioavailability of approximately 70-80% due to first-pass metabolism. Peak plasma concentrations occur within 1-2 hours after oral dosing. In cats, oral bioavailability is lower (approximately 50-60%) and more variable. Injectable formulations provide immediate systemic availability.

Distribution: Ondansetron is widely distributed in the body, with a volume of distribution of about 2.2-2.5 L/kg in dogs. It crosses the blood-brain barrier to a limited extent, but sufficient concentrations reach the CTZ. It is approximately 70-76% bound to plasma proteins in dogs and cats.

Metabolism: Ondansetron undergoes extensive hepatic metabolism, primarily via cytochrome P450 enzymes (CYP1A2, CYP2D6, and CYP3A4). Multiple metabolites are formed, some of which are pharmacologically inactive. In dogs, the half-life is approximately 1-2 hours, while in cats it is slightly longer, around 1.5-3 hours.

Excretion: Metabolites are excreted primarily in the urine (about 50%) and feces (about 45%). Less than 10% of the drug is excreted unchanged in the urine. In animals with hepatic impairment, clearance may be reduced, leading to prolonged half-life.

Half-Life: Dog: 1-2 hours; Cat: 1.5-3 hours; Horse: approximately 2-3 hours (limited data)

Bioavailability: Oral: Dogs ~70-80%, Cats ~50-60%; IM/IV: 100%

Protein Binding: 70-76% in dogs and cats

Available Formulations & Strengths

Oral Tablet 4 mg, 8 mg, 24 mg (PO)

Orally Disintegrating Tablet 4 mg, 8 mg (PO)

Oral Solution 4 mg/5 mL (PO)

Injectable Solution 2 mg/mL (2 mL, 20 mL vials) (IV, IM)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to ondansetron or any component of the formulation
Concurrent use with apomorphine (may antagonize its emetic effect)
Use in patients with congenital long QT syndrome or known QT prolongation (use with caution)
Severe hepatic impairment (dose adjustment may be needed)

Warnings & Clinical Precautions:

Use with caution in patients with cardiac disease, electrolyte abnormalities, or those taking other QT-prolonging drugs
May mask underlying disease causing vomiting; diagnose and treat underlying cause
In cats, oral bioavailability is lower; consider IV route for severe vomiting
Administer IV slowly to avoid hypotension
Safety in pregnant or lactating animals has not been fully established; use only when clearly needed
In food animals, observe withdrawal times; extra-label use requires veterinary oversight

Adverse Effects & Reactions

Common:

Constipation
Headache (in humans; not observed in animals)
Sedation or lethargy (occasional)
Diarrhea (uncommon)

Serious / Severe:

QT prolongation and cardiac arrhythmias (rare)
Hypotension (with rapid IV administration)
Serotonin syndrome (when combined with other serotonergic drugs)

Rare:

Allergic reactions (urticaria, anaphylaxis)
Seizures (in overdose or predisposed patients)
Hepatotoxicity (elevated liver enzymes)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Apomorphine	Ondansetron may antagonize the emetic effect of apomorphine, reducing its efficacy as an emetic.	Moderate
QT-prolonging drugs (e.g., fluoroquinolones, macrolides, antifungals, antiarrhythmics)	Additive risk of QT prolongation and ventricular arrhythmias.	High
Serotonergic drugs (e.g., SSRIs, SNRIs, tramadol, MAO inhibitors)	Increased risk of serotonin syndrome.	High
CYP3A4 inducers (e.g., phenobarbital, rifampin)	May increase metabolism of ondansetron, reducing its efficacy.	Mild
CYP3A4 inhibitors (e.g., ketoconazole, erythromycin)	May increase ondansetron levels, increasing risk of adverse effects.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Sedation
Hypotension
QT prolongation
Arrhythmias
Seizures (in severe cases)
Respiratory depression (rare)

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Monitor ECG and vital signs. Administer IV fluids for hypotension. Use antiarrhythmics if indicated. In severe cases, consider activated charcoal if recent oral ingestion. Provide respiratory support if needed.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Ondansetron is not approved for food animals. Withdrawal times are not established. Extra-label use in food animals requires veterinary oversight and extended withdrawal periods should be considered. Consult FARAD for specific recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), excursions permitted between 15-30°C (59-86°F).

Handling & Special Conditions: Protect from moisture. Keep tablets in original container. Injectable solution should be protected from light and not frozen.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Ondansetron is not FDA-approved for veterinary use, but it is widely used in veterinary medicine as an extra-label drug.

Extra-Label (Off-Label) Use: In the US, extra-label use of ondansetron in food animals is permitted under AMDUCA with veterinary oversight. However, due to lack of withdrawal time data, it should be used with caution and extended withdrawal periods should be considered. In companion animals, extra-label use is common.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Ondansetron is a valuable antiemetic for veterinary patients, particularly for chemotherapy-induced vomiting and postoperative nausea. It is generally well-tolerated, but its use should be reserved for cases where other antiemetics (e.g., metoclopramide, maropitant) are ineffective or contraindicated. In dogs and cats, the typical dose is 0.5-1 mg/kg q12-24h. For severe vomiting, IV administration is preferred. Monitor for constipation and potential cardiac effects, especially in patients with underlying heart disease or electrolyte imbalances. In food animals, use is limited and withdrawal times are unknown; therefore, it should be avoided unless absolutely necessary. Always treat the underlying cause of vomiting.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)