Onion and garlic are the toxic foods that don't behave like toxic foods. There's no vomiting an hour later, no obvious bad night. The damage is to red blood cells, it builds quietly, and the first visible sign often comes several days after the meal - by which point most owners have stopped connecting it to the meal at all.
- No - to both, for both species. Onions, garlic, leeks, shallots and chives are all Allium, and all can cause toxicity.
- Garlic is 3–5 times more toxic than onion, gram for gram.
- Cats are particularly susceptible. MSD reports poisoning in cats after relatively small amounts of onion, including a reported case involving less than a teaspoon of cooked onion.
- Cooking doesn't help. Raw, cooked, dried and powdered forms have all caused it - and the concentrated forms are the ones most often involved.
- Signs are delayed by days. A pet who seems fine that evening tells you nothing.
What onion and garlic actually do
Per the MSD Veterinary Manual, Allium plants contain sulfur-containing oxidants that are released by mechanical disruption - chopping, cooking, chewing - and by gut microbes working on the material afterwards. Once absorbed, they cause oxidative damage to red blood cells, producing oxidative haemolytic anaemia with Heinz body formation, sometimes accompanied by methaemoglobinaemia.
The timeline is the important bit, and it's the reason this poisoning is so often missed:
| Roughly when | What's happening |
|---|---|
| First 1–2 days | Heinz bodies and methaemoglobin begin to rise. Nothing visible. |
| Over the following days | Those changes peak; damaged red cells are removed or destroyed, leading to haemolytic anaemia. |
| Several days after exposure | Clinical signs usually become apparent. |
Clinical signs often appear several days after exposure, although the timing is variable. By the time you can see something is wrong, MSD notes, substantial haemolysis may have already occurred. That's why "he seems fine" is not reassurance here in the way it might be after a stolen sausage.
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How much onion is toxic?
MSD publishes doses at which clinical signs have been reported:
| Species | Reported doses associated with clinical signs |
|---|---|
| Dogs | 15–30 g of raw onion per kg of bodyweight |
| Cats | 5 g of raw onion per kg - or less than a teaspoon of cooked onion |
What that looks like on a chopping board, taking a medium onion at roughly 100–150 g: for a 10 kg dog, that reported range corresponds to roughly 150–300 g of raw onion, or somewhere between one and three onions. This is only a rough illustration, because onion size and composition vary. For a 4 kg cat, it corresponds to about 20 g - approximately a couple of tablespoons of chopped onion - or, in cooked form, less than a teaspoon.
Those figures are the doses at which poisoning has been documented, not a safe allowance underneath. Individual sensitivity varies, and repeated smaller exposures are their own problem, since the oxidative damage accumulates faster than the body replaces the cells.
Why powder is the dangerous form
MSD is specific about this: in cats and dogs, poisoning is most commonly associated with concentrated forms - dehydrated flakes, powders and dry onion soup mixes. Drying removes the water and leaves the oxidants, so a teaspoon of onion powder is not the equivalent of a teaspoon of onion.
That reframes where the real risk sits in a normal kitchen. It isn't usually the whole onion on the counter. It's the things that quietly contain concentrated onion or garlic:
- Gravy granules, stock cubes and packet soup
- Stuffing, and anything from a Sunday roast tray
- Curry, Bolognese, pizza, garlic bread, onion bhajis
- Ready meals and leftovers generally - the category is enormous
- Some baby foods, which is worth knowing if you have both a baby and a dog
They are, often marketed with claims around flea control, immune support or general health. Set against that: MSD lists garlic as 3–5 times more toxic than onion, and the WSAVA Global Nutrition Committee's owner guidance names onions, leeks, shallots, garlic and chives on its list of toxic food ingredients for both dogs and cats. Deliberately dosing a dog daily with a substance that causes cumulative red-cell damage is a decision to make with your vet, if at all - not on the strength of a label.
We haven't converted the garlic figure into a number of cloves anywhere in this article, deliberately. MSD publishes the potency multiplier and not a clove count, and a clove count would read like a permitted dose. There isn't one.
What to watch for
MSD's list of clinical signs, all of which reflect anaemia rather than an upset stomach:
- Lethargy, weakness, exercise intolerance
- Loss of appetite
- Fast breathing and a fast heart rate
- Pale or yellow-tinged gums (jaundice)
- Red or brown urine - haemoglobin from broken-down red cells, which can go on to damage the kidneys
- Collapse
What to do if they've eaten some
- Call your vet now, not in three days. Decontamination is most useful soon after ingestion, before significant absorption has occurred. Treatment is otherwise supportive and may include oxygen, intravenous fluids when indicated, and, in severe cases, a blood transfusion. Early veterinary assessment is preferable to waiting for severe anaemia or collapse.
- Tell them the form and the amount. Raw onion, cooked onion, gravy granules and garlic powder are not the same exposure, and the vet's risk assessment depends on knowing which.
- Don't try to make your pet sick at home. Inducing vomiting is a clinical decision with real risks, and the drugs used are veterinary ones.
- Say so even if it's been a few days. Anaemia developing several days later can still be treated, so don't assume you've missed the window for veterinary care - and the history is what points the vet at the right diagnosis quickly.
In the UK, Animal PoisonLine (01202 509 000) is a 24-hour service for owners, run by the Veterinary Poisons Information Service; there's a per-call fee. In the US, the ASPCA Animal Poison Control Center is on (888) 426-4435, 24 hours, also for a fee.
Cats: the most susceptible species
MSD ranks cats above dogs for Allium sensitivity, and the reported doses show it - poisoning has been reported at around 5 g/kg of raw onion, and in one reported case after less than a teaspoon of cooked onion. For a cat, the everyday risks are licked plates, gravy, and anything from a roast; a cat doesn't need to raid the vegetable rack to get there.
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Her review is dated 14 September 2026 and her comments are applied in full. Dr de Klerk qualified from the Royal Veterinary College in 2014 and is a member of the Royal College of Veterinary Surgeons (reg. 7088779). This article is general information, not a diagnosis, and doesn't replace veterinary care. If your pet may have eaten onion or garlic in any form, contact your vet or a poison line straight away.