Onion — Food safety facts
Do not feed · Cats · full breakdown on the linked food page
Toxic. Damages red blood cells (hemolytic anemia). Powdered onion is concentrated. Danger can appear 1–5 days later.
Dogs and cats carry the same rating (Do not feed), so the notes below hold for either pet.
Onion should not be given to cats at all. Even a small amount can do harm, so there is no portion small enough to be safe.
Do not induce vomiting and do not give milk or oil to “dilute” it. Note the food and roughly how much was eaten, then call your vet or a pet poison hotline and follow their advice on whether to bring the pet in straight away.
Why these cautions matter
Garlic and onion damage a pet's red blood cells and can bring on anaemia. Signs are often delayed for several days, so looking fine straight afterwards does not rule it out.
Onion has no place in everyday feeding: keep the rest of the day's food unchanged after this and watch the litter box and how quickly they finish the next meal.
The full verdict, portion guidance and serving notes are collected on the Onion food facts page.