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What is Onion Poisoning?

The longer you wait to take your cat to a veterinarian, the worse his symptoms may become. To avoid complications, take your cat to a veterinarian as soon as possible after you spot the symptoms of onion poisoning.

Many people use onions to add flavor to their favorite dishes, but unfortunately, onions are toxic to various animals, including cats. In fact, it is believed that cats are more sensitive to the toxins of onions than other animals such as dogs or horses. Even a small amount of the bulb, flower, or stem of an onion can trigger a response in your cat. Once it is consumed, the onion causes oxidative damage to your cat’s red blood cells, which impacts the bloodstream’s ability to transport oxygen to different organs in the body. This condition, known as Heinz body anemia, can be incredibly dangerous.

Symptoms of Onion Poisoning in Cats

Some cats begin to experience symptoms of onion poisoning soon after ingestion, however other cats may not exhibit any symptoms for several days. Some of the most common symptoms to look for include:

  • Excessive drooling
  • Vomiting
  • Diarrhea
  • Pale colored gums
  • Physical collapse
  • Pain in the abdomen
  • Weakness
  • Lethargy
  • Elevated heart rate
  • Elevated respiratory rate

Causes of Onion Poisoning in Cats

Onion poisoning occurs after a cat consumes onion, which can be poisonous in both fresh and dried forms. Every part of the onion, including the bulb, flowers, and stem, are toxic to cats. 

An onion can cause oxidative damage to red blood cells, known as Heinz body anemia, making it impossible for the cat’s bloodstream to transport oxygen to different parts of the body. 

Diagnosis of Onion Poisoning in Cats

If you see your cat eating onion or if you notice any of the symptoms above, take him to a veterinarian as soon as possible for treatment. Talk to the vet about the symptoms you have observed, when they began, and if possible, when your cat consumed onion. 

The vet will begin by taking basic tests to look at the cat’s overall health. These tests include a complete blood count, blood chemistry profile, and urinalysis. These tests will show an abnormally low level of red blood cells, and when a blood sample is analyzed closely, the vet should be able to spot Heinz bodies, which indicates the cat is suffering from hemolytic anemia.  

A number of different conditions can cause hemolytic anemia, so the presence of Heinz bodies does not always lead directly to a diagnosis of onion poisoning. The vet will rely heavily on the symptoms you have described when making a diagnosis, so be as clear and informative as possible.

Treatment of Onion Poisoning in Cats

Treatment will begin immediately following the diagnosis of onion poisoning. The vet will need to induce vomiting to remove any of the onion that remains in your cat’s stomach cavity. To do so, a 3% hydrogen peroxide solution will be administered orally. Following the induced vomiting, the vet may also administer activated charcoal, which will absorb any toxins that still remain in your cat’s stomach cavity. A gastric lavage, which is a type of stomach wash, may also be performed to flush out your cat’s stomach.

If your cat’s skin has also come into contact with an onion, the vet will thoroughly bathe and scrub your cat to remove any toxins lingering on the skin. 

Many cats become dehydrated either before or during treatment because of the vomiting and diarrhea. If this happens to your cat, the vet will need to provide him with fluids via an IV. 

In extreme cases, the cat may have already lost too many red blood cells by the time you take him in for treatment. If this has happened to your cat, a complete blood transfusion may be necessary.

Recovery of Onion Poisoning in Cats

The sooner you take your cat to a veterinarian for treatment, the better his chances are of making a full recovery. If your cat has already suffered severe red blood cell loss by the time he receives treatment, his chances of recovering are much lower.

The vet may keep your cat following treatment to ensure he has been stabilized and no longer needs IV fluids or respiratory support. Once your cat has been released to you, discuss his diet with your vet. It’s important to remove any foods that may contain onion from your cat’s diet. In the days following treatment, the vet may recommend you stick to softer foods to avoid upsetting the cat’s sensitive stomach. Keep your cat comfortable and calm while he regains his strength over the next several days.

Make sure your cat does not have access to any onions. If you keep these out on your kitchen counter for cooking, put them in a safe place where cats cannot come into contact with them.

Onion Poisoning Questions and Advice from Veterinary Professionals

Black cat
5 Years
Fair condition
0 found helpful
Fair condition

Has Symptoms

Nothing yet

My cat just ate a piece of an onion off my sandwich and I didn’t realize they were bad for her but then I read that they are toxic and I was wondering what a little piece of onion will do to my cat?

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
2466 Recommendations
Linhs should be alright with the amount consumed, but if you have some activated charcoal I would recommend that you administer that to help absorb any toxins in the gastrointestinal tract; if you notice lethargy, pale gums or anything else concerning visit your Veterinarian. There are various different figures for toxicity but I quote the figure of 5mg/kg (2.5mg/lb) based on the article below: “Cats are considered more sensitive to onion toxicosis than other species are. Exposures > 5 g/kg can result in gastrointestinal upset, oxidative damage of hemoglobin (resulting in hemolysis), Heinz body anemia, methemoglobinemia and secondary renal damage from hemoglobinuria. If anemia develops, it is generally regenerative unless there is a preexisting health condition.”

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Orange tabby
7 Years
Fair condition
1 found helpful
Fair condition

Has Symptoms


My cat licked my bowl that had potato salad in it. There were some onions in the potato salad, but none at the time he licked it and if there was one it was tiny than a centimeter. Is this enough to harm him? He is acting fine.

Dr. Michele King, DVM
Dr. Michele King, DVM
1044 Recommendations
Christopher, thank you for contacting us about Goucher. Onions are toxic over time, causing changes in blood cell production by the bone marrow. You may notice some GI upset, vomiting or diarrhea, from the potato salad, but the small amount of onion shouldn't cause any toxic signs. If he does start to vomit or have diarrhea, please see you regular veterinarian and have him examined.

Thank you.

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British Shorthair
18 Months
Fair condition
1 found helpful
Fair condition

Has Symptoms

hairball and soft stool

Both my young cats had some taramasalata over the last couple of days, which I now realise has onion in it.
One of them had a soft stool this morning and threw up her food, but with a massive hairball in it. Other than that, there are no adverse symptoms. She has been eating and drinking normally again.
Could there be any poisoning or long term damage, even if there are no symptoms now?

Dr. Michele King, DVM
Dr. Michele King, DVM
1044 Recommendations
Thank you for your email. A small amount of onion may cause mild GI issues, and might explain the soft stool and vomiting. Long term ingestion of onions will cause problems with bone marrow. If Blue continues to have soft stool or vomiting, she should see her veterinarian for an exam, to make sure she is okay.

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