How much onion is toxic to a cat?

As little as 5 grams of onion per kilogram of a cat's body weight is enough to cause significant toxic effects. Even a small piece of raw onion can be dangerous for a typical house cat.

What are the symptoms of onion poisoning?

Common signs include lethargy, weakness, pale or bluish gums, decreased appetite, and reddish-brown urine. Note that these symptoms may take 1 to 5 days to develop after ingestion.

Is cooked onion safer than raw onion?

No. Cooking, dehydrating, or processing onions does not destroy the toxic compounds. All forms of onion are dangerous to cats.

My Cat Ate Raw Onion: Triage Guide and Risk Factors

TL;DR: Onion ingestion is toxic to cats and can lead to life-threatening anemia; contact a veterinarian immediately even if your cat currently appears healthy.

What happens to my cat if they eat an onion?

Onions, along with garlic, leeks, and chives, belong to the Allium genus. These plants contain compounds called N-propyl disulfides which cause oxidative damage to a cat's red blood cells. This damage can lead to Heinz body anemia, a serious condition where the red blood cells break down and can no longer carry oxygen effectively throughout the body.

Is it a veterinary emergency if my cat eats onion?

The urgency level for onion ingestion is classified as Medium, as even small amounts are toxic to felines.
Because symptoms of anemia often do not appear for several days, early intervention is critical.
Contact your veterinarian or a pet poison control center immediately rather than waiting for your cat to act sick.

How does taking a photo of the onion help my vet?

Take a clear photo of the remaining onion or scraps next to a common object, like a coin, for scale.
Showing this photo to a veterinary professional helps them estimate the exact volume of toxin ingested.
This information is vital for calculating the risk level based on your cat's weight and determining if emergency treatments, such as induced vomiting, are required.

Clinical Context (Merck Veterinary Manual)

Onions contain thiosulfate, which can cause oxidative damage to red blood cells and result in anemia in cats. Onions are more toxic than garlic. N-propyl disulfide, an oxidant found in all parts of the onion, inhibits red blood cell glucose-6-phosphate dehydrogenase, leading to hemolysis and formation of Heinz bodies. Clinical signs include hemoglobinuria, diarrhea, loss of appetite, jaundice, ataxia, collapse, and possible death if untreated. Pathology may reveal Heinz body anemia; a swollen, pale, necrotic liver; and hemosiderin in the liver, kidneys, and spleen. Symptomatic and supportive care are essential, as there is no specific treatment.

Chapter: Toxicology

Source: The Merck Veterinary Manual, 11th Edition (Page 3133)

Protocol reviewed by Dr. Robert Taylor, DVM

My Cat Ate Raw Onion: Triage Guide and Risk Factors

Is this an emergency?

What happens to my cat if they eat an onion?

Is it a veterinary emergency if my cat eats onion?

How does taking a photo of the onion help my vet?

Clinical Context (Merck Veterinary Manual)

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