Can cats eat Onions?
No. Onions are toxic to cats in any form — raw, cooked, fried, powdered, or dehydrated. They damage red blood cells and cause haemolytic anaemia.
If your cat has just eaten onions
- Move your cat away from any remaining onion. Do not try to make them vomit at home — this is dangerous in cats and rarely works.
- Note how much was eaten and when, and keep the packaging or a photo of the plant/substance if you can.
- Call your vet immediately, even out of hours. Tell them your cat's weight, what they ate, and when.
- If you can't reach your vet, call Animal PoisonLine (01202 509000) — paid triage, 24/7 for guidance. They can advise on urgency and route you to emergency care.
What's the full picture?
Onions contain compounds called N-propyl disulfide and thiosulphates that damage feline red blood cells by causing oxidative injury. The damaged cells are then destroyed by the body, leading to haemolytic anaemia. Cats are roughly twice as sensitive as dogs to onion toxicity.
Toxicity is dose-dependent: as little as 5g of onion per kilogram of body weight can cause clinically significant changes. For an average 4kg cat, that's about 20g — less than a quarter of a medium onion. Cooking does not destroy the toxic compounds; onion powder is actually more concentrated and more dangerous per gram than fresh onion.
The bigger risk is chronic low-dose exposure. A cat repeatedly fed table scraps containing small amounts of onion (in gravy, stews, stock cubes, baby food, pizza, Chinese takeaway) can develop anaemia over days or weeks without any single 'emergency' moment.
Allium toxicosis in cats is well-documented in UK veterinary literature, including BSAVA guidance and International Cat Care clinical resources. The mechanism — oxidative damage to red blood cells — is the same across the whole allium family (onions, garlic, leeks, chives, shallots).
Symptoms to watch for
Hidden sources you might not think of
- Gravy and stock cubes (OXO, Knorr)
- Baby food (many contain onion powder)
- Chinese and Indian takeaways
- Pizza toppings and pasta sauces
- Crisps with onion flavouring
- Stuffing mixes and ready meals
Safer alternatives
- A small piece of plain cooked chicken breast (no seasoning)
- Commercial cat treats formulated for feline nutrition
- Plain cooked white fish (no bones, no seasoning)
Questions owners ask
My cat licked gravy — is that enough to cause harm?
A single lick of onion-based gravy is very unlikely to cause acute illness in a healthy adult cat, but it's not nothing. If your cat regularly gets gravy, stop — the cumulative dose is the real danger. Watch for pale gums, lethargy, or dark urine over the next week.
Is cooked onion safer than raw?
No. Cooking does not break down the compounds responsible for toxicity. Dehydrated onion (in powders, stock cubes, and crisp seasonings) is actually more concentrated and more dangerous gram-for-gram than fresh onion.
How much onion is fatal to a cat?
There is no single 'fatal dose' — severity depends on the cat's size, health, and whether exposure is one-off or repeated. As a guide, 5g per kg body weight causes measurable red blood cell damage. For a 4kg cat that's around 20g. Don't try to measure — treat any deliberate onion consumption as a vet conversation.
Related
About this guidance
Every entry on this site is compiled from published UK veterinary toxicology sources — International Cat Care, Veterinary Poisons Information Service (VPIS) references, RCVS-registered practice materials, and peer-reviewed feline medicine literature. Where the evidence is mixed, we err on the cautious side because cats are unusually sensitive to many common substances that are harmless to humans and even to dogs.
This is general information written for UK cat owners. It is not personalised veterinary advice for your specific cat, their age, weight, medical history, or the exact exposure you're dealing with. If your cat has eaten something or is unwell, call your vet first. The Animal PoisonLine on 01202 509000 is available 24/7 for a small fee and can tell you whether an emergency visit is needed.
Entries are reviewed and updated as new research emerges. Spotted an error? Let us know — corrections are investigated and applied within 24 hours. For more context on how we work, see about and our full disclaimer.
Last reviewed: · By the What Can My Cat Eat? editorial team