Onions poisoning in dogs and cats
Toxic agent: N-propyl disulfide and related organosulfur compounds · Reviewed 2026-08-18
Phone a vet now, before reading on. Have ready: your pet's weight, roughly how much was eaten and when, and the packaging if there is any. ASPCA Animal Poison Control (+1 888 426 4435), Pet Poison Helpline, or Animal PoisonLine in the UK.
Onions destroy red blood cells in dogs and cats, and the damage is delayed — a pet can look completely normal for two or three days and then become anaemic.
What it actually does
N-propyl disulfide oxidises haemoglobin, which clumps into Heinz bodies. The spleen recognises the damaged cells as defective and removes them, so the animal becomes anaemic days after eating. Cooking does not destroy the compound, and dehydration concentrates it — onion powder is far stronger than the same weight of raw onion.
How much matters
The commonly cited threshold is 0.5% of body weight, which is a smaller amount of onion than it sounds:
| 5 kg dog | roughly 25 g of onion — a few rings |
|---|---|
| 25 kg dog | roughly 125 g — about half a medium onion |
| Onion powder | several times more potent per gram than raw onion |
| Cooked, fried, caramelised | no safer than raw |
Repeated small amounts accumulate, which is why table scraps and gravy matter more than a single stolen ring. Japanese breeds — Akita, Shiba Inu — carry red cells that are more vulnerable, and cats are more sensitive than dogs across the board.
What you will see, and when
Onset: Vomiting within hours; anaemia at 1–5 days. Typical course: Recovery over one to two weeks as new red cells are made.
- HoursVomiting, drooling, abdominal discomfort, mouth irritation
- 1–3 daysLethargy, weakness, pale gums, faster breathing
- 3–5 daysOrange or dark red urine, jaundice, collapse in severe cases
What the vet will do
Decontamination if recent. Then it becomes a haematology case: packed cell volume checked repeatedly over several days, a blood smear examined for Heinz bodies, oxygen support and in severe cases a transfusion. There is no antidote, so the treatment is buying time while the marrow replaces the lost cells.
Outlook
Good in most cases. Serious when a large amount was eaten by a small animal, or when a cat is involved.
Dogs and cats are not the same here
Cats are markedly more sensitive than dogs, and cats also groom, so they ingest what lands on their coat. Akitas and Shiba Inus are the breeds to worry about most.
Where it comes from in a normal house
- Raw, cooked and fried onion
- Onion powder in gravy, stock cubes, crisps and baby food
- Onion rings and battered onion
- Soups, curries, casseroles and leftovers generally
The thing people get wrong
“They seemed fine that evening, so we got away with it.”
Onion poisoning is defined by its delay. The evening after is exactly when a poisoned animal looks fine — the anaemia is still two to four days away.
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Get PawLife on the App StoreFrequently asked questions
How much Onions is dangerous?
Repeated small amounts accumulate, which is why table scraps and gravy matter more than a single stolen ring. Japanese breeds — Akita, Shiba Inu — carry red cells that are more vulnerable, and cats are more sensitive than dogs across the board.
How long after exposure to Onions do symptoms start?
Vomiting within hours; anaemia at 1–5 days. Vomiting, drooling, abdominal discomfort, mouth irritation. Recovery over one to two weeks as new red cells are made.
Is Onions worse for cats or dogs?
Cats are markedly more sensitive than dogs, and cats also groom, so they ingest what lands on their coat. Akitas and Shiba Inus are the breeds to worry about most.
Is it true that: They seemed fine that evening, so we got away with it.
Onion poisoning is defined by its delay. The evening after is exactly when a poisoned animal looks fine — the anaemia is still two to four days away.
Related emergencies
Sources
This page is information, not veterinary advice. Toxic doses vary with body weight, age, breed and individual sensitivity, and the figures above exist to explain what a vet is assessing, never to decide that an amount is safe. If you are reading this because something has already been eaten, call a vet.