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My Cat Ate Onion! Our Vet Explains What to Do

Written by: Dr. Joanna Woodnutt BVM BVS (Veterinarian)

Last Updated on July 15, 2024 by Catster Editorial Team

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My Cat Ate Onion! Our Vet Explains What to Do


Dr. Joanna Woodnutt Photo


Dr. Joanna Woodnutt

MRCVS, Veterinarian

The information is current and up-to-date in accordance with the latest veterinarian research.

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You might have wondered if cats can eat onions or if they would want to, and although cats are less inclined to scavenge than dogs, they can find some human foods irresistible. Cats are unlikely to eat raw onion voluntarily. But cooked onions fried with butter, onion gravy, and tasty food seasoned with onion powder may be particularly enticing to your cat.

Onion is often a hidden ingredient in many recipes. It can even be found in some baby food. So, before you intentionally feed human food to your cat, check the ingredients. That way, you can make sure it does not contain anything harmful. However, it can be particularly frustrating if your cat has snatched food without your say-so. If you aren’t sure if any of the foods were harmful, call your veterinarian, who will be able to help.

If your cat has eaten onion or food with onion added to it, here’s what to do.

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My Cat Ate Onion – What Do I Do?

As onions can cause harm to your cat if eaten, you will need to take certain steps to ensure your cat receives any treatment they might need.

Step-by-Step Guide on What to Do If Your Cat Ate Onion
  • Remove any remaining onion from your cat’s reach so they do not eat anymore. If it is safe, retrieve any food from your cat’s mouth. Take care not to get bitten or scratched.
  • Check what your cat has eaten, how much, and at what time. This information will be vital history to tell your veterinarian.
  • Check if your cat seems unwell or has had any vomiting or diarrhea.
  • Call your veterinary clinic and tell them what has happened. If your veterinary clinic is not open, you need to call the emergency line or your clinic’s emergency provider. Your cat may need to be seen urgently by a veterinarian if the amount of onion eaten is considered toxic or if your cat is showing signs.
  • Safely and calmly transport your cat to the veterinary clinic if instructed to do so by your veterinarian. Bring any relevant information to the clinic, like food labels, for example.
  • Do not try to make your cat vomit. Do not offer your cat food or water until you have spoken to your veterinarian.

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Can Cats Eat Onions?

Onions are part of a group of plants called alliums. Edible alliums, including garlic, chives, shallots, spring onions, and leeks, are commonly found in most home kitchens. All alliums can potentially harm your cat if eaten, and onions are no exception. Onion poisoning in cats and dogs is not uncommon in the veterinary clinic.

In the kitchen, onions that are raw, cooked, powdered, or in granules are all toxic to your cat, even when mixed in food. All varieties and colors are also a risk. So, it is vital to keep an eye on your feline companion when you are cooking and not leave scraps lying around where your cat can reach them. It may be easier to keep your cat out of the kitchen when you are cooking and away from the table when eating, as cats are far more agile than most dogs!

Growing vegetables at home is becoming a popular pastime, and onions are often a favorite choice. It is important to know that even the stalk, flowers, seeds, and leaves can be hazardous to your cat, not just the onion bulb. Some cats love to chew on plants in the garden, and seeds can get stuck to their fur if they walk by onion beds. So, if you are growing onions or other alliums in your garden, make the area catproof.

red onions and powder
Image Credit: hemro, Shutterstock

Can Cats Have Onion Powder?

No, cats cannot have onion powder. Onion powder is dehydrated onion, so a smaller amount of onion powder may be more toxic when compared to the same weight of raw or cooked onion. Onion powder may even be a greater risk since it can be added in large quantities to food as a seasoning. The danger comes from how easy onion powder is to disguise in foods that are tempting to your cat.

Can Cats Have Cooked Onions?

No, cooked onions are also toxic if eaten by your cat. Like onion powder, they may be more hazardous because of the way they are often prepared. Cats love butter, and gravy, which are two common ways to prepare cooked onions. A cat will not stop to wonder if there is anything harmful once they have a taste for it. This could mean cats may consume more than they would if faced with strong-smelling raw onions.

How Much Onion Is Toxic to Cats?

The more onion your cat eats, the bigger the risk of toxicity and severe illness. Evidence suggests that 5 grams of onion per kilogram of body weight or 0.5% of your cat’s total weight is enough to cause harm. If your cat eats this amount, signs of onion toxicity are likely to appear. The lethal dose depends on each case, so we cannot say exactly how much onion kills a cat.

Onion toxicity in cats can develop if they eat a large portion of onion at once or smaller amounts over a longer time frame. The effect of onions on your cat’s body can add up over time. This can sometimes make onion poisoning more challenging for your veterinarian to diagnose since the event may have happened weeks before the signs appear.

vet holding a cat
Image Credit: Yana Vasileva, Shutterstock

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What Are the Signs of Onion Poisoning in Cats?

Signs of onion poisoning may occur within hours, days, or weeks after your cat has eaten onion. At first, your cat may have a stomachache, vomiting, and diarrhea. If your cat has severe vomiting or diarrhea, they may go off their food and become dehydrated. Dehydration is serious and often needs veterinary treatment to correct. Even if unrelated to your cat eating onion, any vomiting or diarrhea that persists for more than 24 hours should be discussed with your veterinarian.

Onions can damage your cat’s red blood cells and cause anemia. Red blood cells carry vital oxygen around your cat’s body to the organs and muscles to keep them healthy. Anemia occurs when the number of normal red blood cells in your cat’s body drops to levels that negatively affect their overall health.

When your cat eats onion, anemia can take time to develop as the onion’s effects builds up over time. It is a serious and life-threatening condition if left untreated. Unfortunately, the longer your cat waits for treatment, the worse their signs will likely become.

Signs of onion poisoning in cats are often related to anemia, and include:
  • Lethargy
  • Rapid breathing
  • Pale or yellow gums
  • Poor appetite
  • High heart rate
  • Eating unusual things or licking the floor (called pica)
  • Vomiting or diarrhea

If your cat has any of these signs, even if you are unsure if they have eaten onion, you should contact your veterinary clinic for advice.

How Do You Treat Onion Poisoning in Cats?

The treatment for onion poisoning in cats is usually symptomatic, as there is no antidote. Instead, your veterinarian will need to examine your cat and take a thorough history so they can give the right treatment.

a siamese cat getting examined by a vet at the clinic
Image Credit: Sergey Nivens, Shutterstock

What Will the Veterinarian Do?

The treatment plan will depend on when your cat ate the onion or whether your cat has toxicity signs. No test will tell your veterinarian if your cat ate onion, so you must tell the clinic what your cat ate, how much, and when.

If you do not know all or any of the information, do not worry. Your veterinarian will identify if your cat has any signs of being unwell and get them the best treatment.

Investigations and treatment for onion toxicity or anemia can include any of the following:
  • An examination
  • Injection to make your cat vomit
  • Activated charcoal
  • Blood tests
  • Ultrasound or x-ray
  • Intravenous fluids
  • Anti-nausea medication
  • Blood transfusion

Once at the clinic, your veterinarian will examine your cat. They will look for external signs of anemia like pale gums, a fast heartbeat or rapid breathing, and a general appearance of dullness. This will help them decide what to do next.

If your cat appears well and is not showing any signs of toxicity, your veterinarian may give your cat an injection to make them vomit. This scenario usually happens if you got your cat to the clinic soon after eating the onion or it isn’t causing visible effects on your cat’s body. The injection is given under your cat’s skin and takes effect within 10-15 minutes.

Once your cat has brought back the onion, as long as they are well, you may be able to take your cat home again. Your veterinarian will likely ask you to monitor your cat at home for any illness that develops.

If your cat is not well on arrival at the veterinary clinic, they may need more tests and investigations. Your cat may have a blood sample taken. A blood sample will tell your veterinarian if your cat is anemic and how bad the anemia is. Anemia in cats can be common and is not only caused by onion toxicity. So, your veterinarian will also want to look for other diseases that could explain your cat’s signs. This could include an ultrasound or x-ray if you are unsure if your cat ate onion.

If severely anemic, some cats will need a blood transfusion. Your cat may need to stay at the clinic if they are unwell or need a blood transfusion. Your veterinarian will want to check that your cat’s red blood cell count improves over time, so you may need to repeat visits to the clinic.

Other treatments are purely supportive and include intravenous fluids and anti-nausea medication. Intravenous fluids will help rehydrate your cat and support their circulation. Some cats will receive activated charcoal, which is given by mouth. This reduces the absorption of toxins from your cat’s gut into their blood.

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Will It Be Okay If My Cat Ate Onion?

If your cat ate a tiny piece of onion once, it may not cause toxicity, but they could still get an upset tummy. However, it depends on your cat’s size compared to how much onion they have eaten. Your cat may be more vulnerable to the effects of onion poisoning if they suffer from another illness, such as kidney disease.

If your cat eats a large portion of onions, the outcome will depend on how quickly you get to the veterinary clinic. The effect of eating onion can get worse over time. If your cat develops onion toxicity, the prognosis depends on how well they respond to treatment.

Cats can replace their red blood cells in around 70 days. Mild anemia resulting from onion poisoning will often improve with symptomatic treatment alone. Unfortunately, the more severe your cat’s anemia, the worse the prognosis. That is why swift treatment is critical.

So, if your cat has eaten onion or anything that has onion in it, contact your veterinarian right away for the best chance at recovery.

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Raw, cooked, or powdered onion can cause toxicity in your cat if they eat it. The most concerning side effect of eating onion is anemia, which can be fatal if severe. You must call your veterinarian for advice if you see or think your cat has eaten anything that contains onions, garlic, or other alliums. Call your veterinary clinic urgently if your cat is unwellG or has any signs described above. Do this even if you are unsure if they have eaten onion, as you will increase the likelihood of a good outcome for your cat.

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Featured Image Credit: H Ko, Shutterstock

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