Can cats eat onions?
No — onions are toxic to cats in every form: raw, cooked, fried, dried or as powder. Their sulphur compounds destroy red blood cells and cause anaemia.
Which substance is dangerous?
Onions — like all alliums — contain sulphur compounds (N-propyl disulphide and related) that oxidatively damage feline red blood cells. The cells burst; the result is haemolytic anaemia. Cats are considerably more sensitive to this than dogs, and cooking, drying or frying does not destroy the toxins. That makes seasoned leftovers, gravies, baby food with onion powder and stock particular traps.
How much is dangerous?
Even about 5 g of onion per kilogram of body weight can trigger poisoning — for a 4 kg cat that is a fraction of one onion. Small, repeated doses (onion powder in food scraps) add up to the same damage.
Symptoms of poisoning
Vomiting and diarrhoea early on; after one to several days the anaemia shows: weakness, fast breathing, pale or yellowish gums, dark urine, loss of appetite. The delay makes onion poisoning treacherous.
Emergency: what to do?
Onion eaten? Call the vet even without symptoms — the anaemia develops with days of delay. State what was eaten and how much. Blood checks over the following days may be needed.
Related foods
Frequently asked questions
Are cooked onions also toxic?
Yes. Cooking, roasting or drying does not destroy the sulphur compounds — every form of onion is toxic to cats.
Is onion powder in food scraps a real risk?
Yes, a particularly common one: gravies, seasoned meat and stock often contain onion powder. Small regular doses add up to the same anaemia.
How fast do symptoms appear?
Digestive upset early, but the dangerous anaemia often only after one to several days — pale gums, weakness, dark urine.
My cat ate seasoned leftovers — what should I do?
Call the vet and describe the dish. If onions or garlic were involved, blood monitoring over several days may be advisable.
Important note
This article never replaces veterinary advice. If you suspect poisoning or symptoms persist, contact your vet or the emergency veterinary service immediately.
Sources: Veterinary toxicology: allium poisoning (haemolytic anaemia) in cats. Last updated: 08/2026.
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