Ondansetron

What is this medication for?

• Severe/intractable vomiting (parvo enteritis, pancreatitis, chemotherapy) when conventional antiemetics fail
• Intractable vomiting — controversial, some avoid entirely

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Well tolerated overall
• Constipation, drowsiness, extrapyramidal signs (head shaking), arrhythmias, low blood pressure (<10% incidence (human data))

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Hypersensitivity to 5-HT3 antagonists
• has or has had: May mask ileus/gastric distension — do not substitute for NG decompression when indicated; hepatic dysfunction; use caution in MDR1-mutant dogs (P-glycoprotein substrate in humans, unconfirmed in dogs)

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Apomorphine
  • QTc-prolonging drugs (amiodarone, cisapride, halothane, isoflurane, sotalol)
  • Tramadol
  • Metoclopramide

Storage and handling

2–30°C, protect from light. Keep solution pH <6 to prevent precipitation. Compounded oral prep stable ~42 days if pH kept low.

Good to know

• Oral bioavailability in dogs is under 10% — if using ondansetron for a dog, into a vein or under the skin administration is far more reliable than oral dosing despite oral being commonly prescribed.
• In cats, the longer half-life via the under the skin route makes under the skin dosing preferable to into a vein/by mouth for sustained effect.
• Reserved for intractable vomiting when maropitant/metoclopramide are insufficient — not typically first-line given cost and administration-route limitations.