Ondansetron for dogs: Dosage and safety

Ondansetron (Zofran) is a powerful anti-nausea drug that prevents and reduces nausea and vomiting. In human patients, it’s used to treat vomiting and nausea due to cancer chemotherapy or radiation therapy. Veterinarians commonly use ondansetron for chemotherapy, but its use in dogs is “off-label.” Vets may also use it in cases of severe or repeated vomiting. Ondansetron belongs to a small family of drugs called 5-HT3 receptor antagonists. Also called serotonin antagonists, these drugs reduce nausea and vomiting by blocking the chemical that transmits nerve signals in the gastrointestinal tract and the part of the brain that controls nausea called the chemoreceptor trigger zone (CRZ). Signals between the brain and the gastrointestinal tract are weakened, so the nausea reflex doesn’t activate. 

RELATED: Save up to 80% on ondansetron

What is ondansetron used for in dogs?

Veterinarians commonly use ondansetron to treat severe or frequent nausea and vomiting that can’t be managed with other anti-nausea drugs. Nausea-inducing chemotherapy is a common reason veterinarians give dogs ondansetron. However, ondansetron is a first-line treatment for severe or frequent vomiting due to other medical conditions, including pancreatitis or severe intestinal parvovirus infections. They might also use it as an add-on drug for severe diarrhea. Research suggests ondansetron may be useful at treating nausea in dogs with inner ear problems (vestibular disease). Still, it hasn’t become a standard extra-label treatment for that condition in veterinary medicine.

Chemotherapy

As with humans, dogs often experience nausea and vomiting after chemotherapy doses. Veterinarians can control these side effects by giving the dog ondansetron before and after each chemotherapy dose. Treatment usually involves a single dose or a handful of doses administered over a few days. 

Vomiting

Most of the time, ondansetron is not a first-line drug if a dog is nauseous or vomiting unless the vomiting is severe and continuous. In those cases, ondansetron or similar drugs are often the first choice for treatment. It’s important to treat severe or frequent vomiting with potent antiemetics as early as possible to stop the vomiting. 

Diarrhea

Ondansetron can also help relieve diarrhea in dogs. That’s because the way ondansetron works in quelling nausea also slows down intestinal contractions and reduces the water content of stools. Constipation is a side effect of ondansetron treatment, giving it some effectiveness in relieving diarrhea. For diarrhea, it’s only used as an adjunct treatment, that is, as an add-on drug to another antidiarrheal medication.

Is ondansetron safe for dogs?

Ondansetron has relatively few side effects in dogs, severe side effects are rare, and treatment is usually short-term. However, no medicine is 100% safe in all dogs.

Ondansetron should never be given to dogs who:

• Have had an allergic reaction to ondansetron or similar drugs like granisetron or dolasetron
• Have a large intestine that’s not working (ileus)
• Have an enlarged stomach (gastric distention)

Ondansetron may be risky in dogs with liver disease. The liver breaks down ondansetron. If it’s not working well, the drug can build up in the dog’s blood and cause side effects. In these cases, the veterinarian will probably reduce or space out doses over longer times. 

Ondansetron can be safely given to dogs with kidney disease. 

Side effects of ondansetron in dogs

The most common side effects of ondansetron in dogs are:

• Constipation
• Sleepiness and drowsiness
• Head shaking 
• Low blood pressure (hypotension)

In one study, Ondansetron used in very high doses was associated with a change in the electrical signals within the heart. No changes in blood flow or symptoms were noted in these dogs, but vets may use lower doses in dogs who have underlying heart disease. Interactions of ondansetron with other pet meds

All drugs interact with other drugs, so the veterinarian should know about all prescription drugs as well as any over-the-counter medications, supplements, or at-home remedies given to the dog, like CBD. Most other medications and supplements are safe with ondansetron, but it’s important to check.

Ondansetron has relatively few drug interactions that veterinarians worry about. These include:

• Drugs that affect the heart’s rhythm

Ondansetron could add to the effects of drugs that change the heart’s rhythm.

• The narcotic painkiller tramadol 

Both tramadol and ondansetron may be less effective when used together

• Apomorphine, a drug that induces vomiting (emetic) in dogs

Ondansetron dosage for dogs

Ondansetron doses vary based on the treated condition and whether the medication is given orally or injected. 

Dosages also vary depending on the source. Merck Veterinary Manual gives a standard dose of 0.1–0.2 mg/kg when given orally and 0.1–0.15 as an intravenous injection. These dosages appear elsewhere in veterinary literature.

On the other hand, Plumb’s Veterinary Drug Manual lists the following ondansetron dosages for dogs:

• For nausea or vomiting due to chemotherapy: 0.5–1 mg/kg given by mouth every 8–12 hours OR 0.1–0.5 mg/kg intravenous infusion every 8 hours or 30 minutes before administering cisplatin
• To control severe or frequent vomiting: 0.1–1 mg/kg intravenous injection every 8–12 hours 
• As an antiemetic: 0.1–1 mg/kg given by mouth every 12–24 hours OR 0.1–1 mg/kg IV injections every 6–12 hours

Ondansetron is often used for chemotherapy or serious vomiting, so there’s a good chance a veterinary professional will administer most or all the doses. Pet parents sent home with ondansetron will be given either human versions of ondansetron or oral formulations prepared by a compounding pharmacy. Dosage forms include tablets, orally disintegrating tablets, and liquid oral solutions. A compounding pharmacy can also make a transdermal gel to apply to the dog’s skin, but research suggests this may not be an effective way to give animals ondansetron. 

Can dogs overdose on ondansetron?

Do not give a pet too much ondansetron. Too much ondansetron has caused sudden, short-lived blindness, low blood pressure, and severe constipation in human patients. There is also a risk of serotonin syndrome, a potentially serious complication of taking drugs like ondansetron that affects serotonin levels. Call an animal poison control center or a veterinarian if too much ondansetron is given to a dog. Take the dog to a veterinarian or animal hospital if the dog starts showing unusual symptoms.

Can I give my dog ondansetron every day?

Ondansetron can be given to a dog daily for the length of time prescribed by the veterinarian. That probably won’t be for very long. In humans, ondansetron is a short-term treatment. In people, it’s often given in a single dose or a handful of doses over one or two days. 

How to give your dog ondansetron

Among pet meds, ondansetron is relatively easy to give to a dog because the treatment only lasts a few days. 

• Follow all the veterinarian’s instructions. 
• Do not miss doses. The dog will usually need a dose every eight or 12 hours.
• Since ondansetron is given to dogs who are nauseous or vomiting, the dog may vomit up a dose. The dose may not be to blame. Ask the veterinarian what to do in case this happens.
• Ondansetron can be given with or without food.
• If you have problems giving your dog tablets, you can mix the tablet with food. A veterinarian may also be able to supply you with ondansetron in an easier dosing format.
• If you are using the oral solution:
  • There is no need to shake the bottle before measuring a dose.
  • Use a calibrated syringe to measure the dose—the veterinarian should supply you with one.
• If you use ondansetron gel, wear gloves when applying it to the dog’s skin.
• Store ondansetron at room temperature, protected from light.
• There is no need to taper the dose when this drug is discontinued.

Ondansetron alternatives for dogs

Veterinarians have a broad range of anti-nausea drugs to choose from. They span many different drug classes, so they work in very different ways. Veterinarians will choose the best medicine for a dog’s particular situation.

Ondansetron works very fast, dramatically reducing the incidence of vomiting and signs of nausea, such as lip licking in a dog. For this reason, ondansetron is a drug of choice to treat severe or frequent vomiting. Other top-line choices include dolasetron and maropitant. All are powerful anti-nausea drugs. When a dog has severe or frequent vomiting, it’s important to use these potent anti-nausea drugs as early as possible. 

For nausea and vomiting due to chemotherapy, metoclopramide or maropitant are the standard treatment options in addition to ondansetron. 

Other anti-nausea drugs are usually not used for severe vomiting or chemotherapy—at least, not as a first choice. They are more appropriate for less urgent cases of nausea and vomiting, such as motion sickness or stomach problems. 

Summary

Ondansetron is a first-choice treatment for severe or frequent vomiting in dogs and is commonly used to control chemotherapy-induced nausea and vomiting. It rapidly and dramatically reduces vomiting and increases the dog’s comfort. It has few side effects, and treatment lasts for a short time. If you are sent home with ondansetron tablets or liquid, give each dose as instructed and on time to keep the dog comfortable.