Call ASPCA Poison Control: (888) 426-4435 or go to your nearest emergency vet. Do not wait for symptoms.
Onions (and all Allium species — garlic, leeks, shallots, chives) are toxic to cats. They damage red blood cells, causing a condition called Heinz body anaemia, which can be fatal if enough is consumed or if exposure is repeated over time.
Why Is Onion Toxicity Toxic to Cats?
Organosulphur compounds from onions oxidise haemoglobin in red blood cells, forming Heinz bodies. These abnormal red blood cells are destroyed by the spleen, leading to haemolytic anaemia. Cats are more susceptible than dogs because feline haemoglobin has more oxidation-susceptible cysteine residues.
Cats are particularly vulnerable due to their unique metabolic profile — they lack certain liver enzymes that other mammals use to process many compounds, making them more sensitive to a range of toxins than dogs or humans.
How Much Onion Toxicity Is Toxic to Cats?
These thresholds are general guidelines. Always use your cat's actual weight and contact Poison Control for a personalised risk assessment.
| Cat weight | Mild signs | Serious signs | Potentially fatal |
|---|---|---|---|
| 3 kg cat | Concerning: >5g raw onion | Serious: >15g raw onion | Potentially fatal: >30g raw onion |
| 4 kg cat | Concerning: >7g raw onion | Serious: >20g raw onion | Potentially fatal: >40g raw onion |
| 5 kg cat | Concerning: >9g raw onion | Serious: >25g raw onion | Potentially fatal: >50g raw onion |
ℹ️ ⚠️ Cooked onion is still toxic — cooking does not destroy the harmful compounds. Powdered onion is more concentrated than raw onion by weight.
Symptoms of Onion Toxicity in Cats
Onset: Days (haemolytic anaemia develops gradually). Watch for these signs:
Treatment window: Symptoms may be delayed 2–5 days after ingestion. Even without immediate symptoms, vet evaluation is recommended for any known onion ingestion.
What to Do If Your Cat Was Exposed to Onion Toxicity
- Stay calm and note the exposure details — what was consumed, how much, and when. This is critical information for the vet.
- Call ASPCA Poison Control immediately — (888) 426-4435. Available 24/7. They will tell you whether emergency care is needed.
- Do not induce vomiting unless specifically instructed by a vet or Poison Control. Incorrect induction can cause additional harm.
- Go to an emergency vet if symptoms are already present or Poison Control advises it. Find a 24-hour emergency vet near you.
- Bring the packaging or a sample of what your cat was exposed to if safe to do so.
Frequently Asked Questions
No. Cooking does not destroy the toxic compounds in onions. Boiled, fried, dehydrated, or powdered onion is all toxic to cats.
As little as 5g of onion per kg of body weight (about 1 teaspoon for an average cat) can cause red blood cell damage. Smaller amounts may cause harm with repeated exposure.
Even small amounts in soups or sauces can contribute to toxicity, especially if your cat eats onion-containing food regularly. Call Poison Control to assess the risk.
Treatment depends on severity. Early ingestion may require induced vomiting. Anaemic cats may need blood transfusions. All cases should be evaluated by a vet.