Ondansetron
Ondansetron is a highly effective, centrally and peripherally acting **antiemetic** belonging to the 5-HT3 receptor antagonist class. Key clinical highlights: * **Refractory Vomiting**: Often utilized as a rescue or second-line antiemetic when conventional therapies (like maropitant or metoclopramide) are insufficient. * **Chemotherapy & Severe Disease**: Considered a gold standard for preventing and treating chemotherapy-induced nausea and vomiting (CINV), particularly with highly emetogenic drugs like cisplatin. It is also widely used in severe cases of **parvoviral enteritis**, **pancreatitis**, and **uremia**. * **Species Considerations**: Generally well-tolerated in dogs. Its use in cats has historically been debated, but it is frequently used in feline practice for intractable vomiting and severe pancreatitis with good clinical success. > **Clinical Pearl**: Ondansetron does not possess prokinetic properties. If gastric stasis or ileus is present, it will not resolve the motility issue and may mask underlying gastric distention.
Mechanism: Ondansetron is a highly selective and competitive **5-HT3 (serotonin type 3) receptor antagonist**. * **Peripheral Action**: Cytotoxic drugs, severe inflammation (e.g., pancreatitis), or viral infections (e.g., parvovirus) cause cellular damage in the GI tract, triggering the release of serotonin from enterochromaffin cells. This serotonin binds to 5-HT3 receptors on vagal afferent nerve terminals, sending emetogenic signals to the brain. Ondansetron blocks these peripheral receptors. * **Central Action**: It also directly antagonizes 5-HT3 receptors located in the **Chemoreceptor Trigger Zone (CTZ)** within the area postrema of the brainstem, preventing central emetic signaling to the vomiting center. * **Net Effect**: Profound inhibition of the vomiting reflex without altering GI motility or causing significant sedation.
Dosing by species
- Empiric dose ยท 0.5 mg/kg IV or PO twice daily. ยท IV/PO ยท q12h
- Anti-emetic for intractable vomiting ยท 0.1-0.15 mg/kg slow IV push q6-12h as needed ยท IV ยท q6-12h ยท as needed
- Antiemetic for adjunctive treatment of severe pancreatitis ยท 0.1-1 mg/kg PO or IV q12-24h ยท PO/IV ยท q12-24h
- All uses (nausea and vomiting) ยท 0.5 mg/kg loading dose followed by 0.5 mg/kg/h infusion ยท IV ยท infusion for 6 hours ยท 6 hours ยท Used when signs are not controlled by other drugs.
- All uses (nausea and vomiting) ยท 0.5-1 mg/kg ยท PO ยท q12-24h ยท As needed ยท Oral bioavailability is considered to be low. Subcutaneous administration is more bioavailable.
- Severe vomiting (including parvovirus or pancreatitis) ยท 0.1-0.3 mg/kg IV given as a slow push every 8 to 12 hours (based on patient response). ยท IV ยท q8-12h ยท Has produced dramatic results in controlling or decreasing frequency of vomiting.
- Chemotherapy-related vomiting ยท 0.5-1 mg/kg PO q8-12h; 0.1-0.5 mg/kg IV over 15 minutes q8h or 30 minutes before cisplatin infusion. ยท PO/IV ยท q8-12h
- Adjunctive therapy of acute diarrhea ยท 0.5-1 mg/kg PO twice daily. ยท PO ยท q12h ยท Antagonizes neuronal 5-HT3 receptors and inhibits Cl- and H2O secretion from intestinal epithelial cells.
- Antiemetic ยท 0.1-0.2 mg/kg IV q6-12h or 0.1-1 mg/kg PO q1224h ยท IV/PO ยท q6-12h (IV) or q12-24h (PO)
Routes of administration
Contraindications
- Hypersensitivity to ondansetron or other 5-HT3 antagonists
- Concurrent use with apomorphine (due to risk of severe hypotension)
- Intestinal obstruction
Adverse effects
- Constipation
- Sedation
- Extrapyramidal clinical signs (head shaking)
- Arrhythmias
- Hypotension
- Headaches (reported in humans)
- Alterations in liver enzymes
- Hypersensitivity reactions (rare)
Drug interactions
- Apomorphine ยท Severe hypotension reported in humans; concurrent use is contraindicated.
- Drugs affecting QTc interval (e.g., amiodarone, cisapride, halothane, isoflurane, sotalol) ยท Theoretically may have additive effects on QTc interval, potentially resulting in serious arrhythmias.
- Tramadol ยท May reduce the efficacy of both drugs (human data); veterinary significance is unknown. ยท moderate
Monitoring
- Clinical efficacy (reduction or cessation of vomiting)
- Hydration status and electrolyte balance (due to underlying vomiting)
- Resolution of nausea and vomiting
- Liver enzymes (with prolonged use)
- Bowel movements (monitor for constipation)
Overdose
Overdoses of up to 10X did not cause significant morbidity in human subjects. If an overdose occurs, treat supportively.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.