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Ondansetron for cats: Dosage and safety

Ondansetron treats nausea and vomiting in cats
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By | Aug. 15, 2024 Medically reviewed by Emma Ryan, DVM
A grey cat sitting with its tongue out: Ondansetron for cats
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Ondansetron is an anti-nausea drug that veterinarians give to cats for severe nausea and vomiting. Ondansetron for cats is an extra-label use of the drug. It’s a powerful antiemetic that works rapidly and dramatically. The drug essentially shuts down the involuntary reflex that causes vomiting. Though powerful, it has few and relatively uncommon side effects. Even so, veterinarians reserve its use for only the most severe feline nausea and vomiting cases. 

RELATED: Save up to 80% on ondansetron

What is ondansetron used for in cats?

Ondansetron is commonly used by veterinarians to treat severe or frequent vomiting and nausea in cats that can’t be managed with other antinausea drugs. The most common reasons ondansetron is needed in cats are chemotherapy, drug treatment, and pancreatitis (inflamed pancreas). 

Ondansetron belongs to a small family of drugs called 5-HT3 receptor antagonists or simply serotonin antagonists. It blocks a nerve-signaling chemical called serotonin in the part of the brain that controls nausea and vomiting. This part of the brain is called the chemoreceptor trigger zone (CRZ) or simply the “nausea center.” Communication between the brain’s nausea center and the gastrointestinal tract is blocked, so the brain can’t activate the vomiting reflex. 

A single dose, usually given intravenously by a veterinarian, can stop vomiting and make the cat feel more comfortable in one to two hours.

Chemotherapy and drug-induced vomiting

Like dogs and humans, cats can get malignant cancers that require chemotherapy. Dogs get cancer at about the same rate as people, but the rate in cats is not as well understood. Veterinarians are also not as knowledgeable about treating feline cancer as they are about treating cancer in dogs.

Ondansetron is a mainstay drug in human medicine to control nausea caused by chemotherapy. Cats and dogs also get nauseous and vomit frequently when put on chemotherapy or other drugs that cause nausea. Veterinary guidelines recommend intravenous injections as needed to treat chemotherapy-induced vomiting. The guidelines do not specify the number of doses, so veterinarians will use the drug as needed. As in humans, doses may be given both before and after chemotherapy.

Pancreatitis

Pancreatitis is inflammation of the pancreas due to a variety of causes. Based on autopsies, veterinarians know that pancreatitis is very common among cats. However, it’s not usually symptomatic. Pancreatitis can be very mild to very severe, but when it’s severe, nausea and persistent vomiting are significant problems. 

Veterinarians primarily use intravenous ondansetron to treat pancreatitis-induced vomiting, but oral dosing is also possible. The drug will be used along with other drugs to treat the pancreatitis.

Vomiting

When cats are vomiting due to other problems, ondansetron is not always the first choice unless the vomiting is intractable. Doses may be given intravenously or by mouth. 

Is ondansetron safe for cats?

Ondansetron is only used for a few doses in severe cases. In those cases, ondansetron is a safe drug for cats that has few side effects.

It is never given to cats that:

  • Have had an allergic reaction to ondansetron or other serotonin antagonists like dolasetron or granisetron 
  • Have a large intestine that is not contracting and moving feces out of the body (ileus)
  • Have an enlarged stomach (gastric distention)

Veterinarians may need to reduce the dose in a cat with liver problems. In humans, doctors only reduce the dose for people with severe liver disease. The degree of liver impairment requiring a dosage adjustment in cats has not been specified in veterinary medicine. 

Cats with kidney disease or chronic kidney disease (CKD) can take regular doses.

Side effects of ondansetron in cats

Though uncommon, possible side effects of ondansetron in cats are:

  • Constipation
  • Sleepiness and drowsiness
  • Involuntary muscle movements like head shaking
  • Low blood pressure (hypotension)

Low blood pressure does not have symptoms, but the cat might get lightheaded, feel dizzy, faint, collapse, or be lethargic.

Interactions of ondansetron with other pet meds

Ondansetron has relatively few major drug interactions. These include:

  • Drugs that cause heart rhythm abnormalities, such as amiodarone, cisapride, or sotalol
  • The narcotic painkiller tramadol

To be safe, tell the veterinarian about all prescription drugs, over-the-counter medications, supplements, and herbal remedies the cat is on. 

Ondansetron dosage for cats

Ondansetron dosages can vary. The standard dosage is 0.5 mg per kilogram of body weight, given twice daily, 12 hours apart. Plumb’s Veterinary Drug Manual recommends that the doses be given intravenously.

Other published ondansetron dosages include:

  • For severe vomiting: 0.1–1.0 mg/kg given as an intravenous infusion every six to 12 hours as needed. 
  • To control vomiting due to severe pancreatitis: 0.1–1 mg/kg given by mouth or as an intravenous injection every 12 or every 24 hours
  • As an antiemetic: 0.1–0.2 mg/kg given by mouth every 12 hours or every 24 hours

Most published dosages recommend intravenous injections for cats so that doses will be administered by a veterinary specialist in a clinical setting. 

Some veterinarians may send a pet owner home with oral ondansetron. In those cases, pet parents can choose between human versions or have the medication custom compounded into a specific dose or flavored oral liquid by a compounding pharmacy. 

Human versions come in several dosage formats, including orally disintegrating tablets and liquid oral solutions. The brand-name version of ondansetron is Zofran, but it’s been discontinued.

Can cats overdose on ondansetron?

An ondansetron overdose can cause serious problems in a cat. In humans, the adverse effects of an overdose include short-lived blindness, low blood pressure, severe constipation, and serotonin syndrome. If you suspect the cat has been given an overdose, call the veterinarian or take the cat to a veterinary hospital. 

Can I give my cat ondansetron every day?

Ondansetron is given to cats on an as-needed basis to stop severe vomiting. In these cases, ondansetron is a safe drug to give to cats and has relatively few side effects.

How to give your cat ondansetron

Most cat parents will not give their cat ondansetron doses. Instead, the veterinarian will inject the medicine in a clinical setting. If the veterinarian sends the cat home with an ondansetron prescription, pet owners can expect to give their cat tablets, liquid, or a topical gel.

Giving doses is relatively straightforward:

  • Follow all the veterinarian’s instructions. 
  • Understand the dosing schedule. Pet parents will dose their cat every 12 or 24 hours. 
  • If you don’t know how to give a cat a tablet, ask the veterinarian or a veterinary technician to demonstrate the best technique.
  • If giving the cat a pill is difficult, a compounding pharmacy may be able to make an ondansetron oral solution or treat for easier dosing.
  • Ondansetron tablets and oral solutions can be given with or without food.
  • If the cat is being dosed with ondansetron oral solution:
    • There is no need to shake the bottle before measuring a dose
    • Use a calibrated oral syringe to measure the dose
  • Since the cat is vomiting, it may vomit the tablet or oral solution. Ask the veterinarian what to do in case this happens.
  • There is no need to taper the dose when this drug is discontinued.

Ondansetron alternatives for cats

Ondansetron is not a first-choice drug for most cases of vomiting and nausea in cats. It’s too powerful for mild cases and doesn’t work for motion sickness. Instead, veterinarians only use it in cats with severe or persistent vomiting.

If a cat cannot take ondansetron, veterinarians have many other choices that can relieve severe vomiting, including:

For less severe nausea and vomiting, veterinarians will rely on milder anti-nausea drugs such as omeprazole, famotidine, sucralfate, mirtazapine, or cisapride. 

Summary

Ondansetron is a powerful anti-nausea drug that rapidly reduces severe and persistent nausea and vomiting in cats. There are few side effects, but veterinarians only use it briefly. Most cats will get their ondansetron doses from a veterinarian as an injection. A cat may be sent home with oral ondansetron. In those cases, pet owners should diligently follow the veterinarian’s instructions. 

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