Poisoning guide
Onions and garlic: the toxicity that shows up three days later
The dog will seem completely fine. Then, two to four days later, the gums go pale. That delay is what makes allium poisoning so easy to miss.
The short version
- One family, one mechanism: onion, garlic, leek, shallot, chive and spring onion all destroy red blood cells.
- Rough oral toxic dose: around 15–30 g of onion per kg of body weight, and roughly 5 g/kg for garlic, which is more concentrated.
- Delayed: signs typically appear 1–5 days later, when enough red blood cells have been lost.
- Powder is worse: dehydrated onion and garlic are 5–10 times more concentrated by weight than fresh.
- Hidden sources cause most cases: stock cubes, gravy, soup, sausages, stuffing and leftovers.
What alliums do to a dog's blood
Onion, garlic and their relatives contain organosulphur compounds — the same chemistry that makes them smell like onions. In dogs, these compounds oxidise haemoglobin, the molecule that carries oxygen, into a form that cannot hold it. The result is a Heinz body: a red blood cell with a visible bulge of damaged protein.
The body responds by removing those damaged cells from circulation. That is fine when a few cells are affected. When a large proportion are damaged, the dog becomes anaemic — short of oxygen-carrying capacity — and the signs appear days later, once the loss has accumulated. This delayed onset is the single most important thing to understand about allium toxicity. The dog that ate onion on Monday may look normal until Thursday.
Cats are considerably more sensitive than dogs, which is worth knowing in a multi-pet household. And because the damage is cumulative, a small amount every day (a spoonful of gravy, a bit of leftover stew) can add up to the same effect as one large exposure.
Dose: fresh versus powdered
| Form | Approximate amount for a 10 kg dog | Notes |
|---|---|---|
| Fresh onion | 150–300 g (1–2 medium onions) | Roughly 15–30 g/kg |
| Onion powder | 15–30 g | Concentrated; a few tablespoons |
| Fresh garlic | 50 g (about 15 cloves) | Roughly 5 g/kg; more potent than onion |
| Garlic powder | 5–10 g | Concentrated; 1–2 teaspoons |
| Leek / spring onion | Similar to onion by weight | Lower sulphur content but still significant |
These figures describe the dose at which anaemia becomes likely, not the dose at which nothing happens. A dog that eats a slice of onion on a burger is probably fine. A dog that eats a bowl of onion soup is not — and the difference is often not obvious from the amount alone, which is why a vet call is the right response to any substantial exposure.
The signs, and when they appear
- Day 1–2: often nothing at all. Possibly mild vomiting or diarrhoea.
- Day 2–4: lethargy, weakness, reluctance to walk, rapid breathing after mild exertion.
- Day 3–5: pale or white gums, dark or reddish-brown urine (from haemoglobin breakdown), collapse in severe cases.
- Recovery: with supportive care, red blood cell counts typically recover over one to three weeks.
Pale gums are the sign to check for at home. Lift the lip and look at the gum above the canine tooth: it should be a healthy pink. A dog with gum colour like white paper needs to be seen immediately.
Where alliums hide in a normal kitchen
Very few dogs eat a raw onion voluntarily. Almost every case involves a prepared food that the owner did not think of as containing onion.
- Stock cubes and bouillon — onion and garlic are near-universal ingredients
- Gravy granules and instant sauces
- Canned soup and ready meals
- Sausages and processed meats
- Stuffing and sage-and-onion mixtures
- Pizza, pasta sauce and curry
- Baby food containing onion or leek
- Some "pet-safe" home-cooked recipes that include garlic as a "natural flea repellent"
If you cook for your dog, the rule is simple: no stock, no gravy, no seasoning blends. Plain water and plain meat are the base of everything.
The garlic myth
Garlic is widely recommended in folk remedies as a flea repellent and immune booster. There is no evidence it repels fleas at any dose, and the dose required to have any measurable effect is inside the toxic range. It is not a safe supplement, and it is not a substitute for a licensed flea treatment.
A dog that gets a spoonful of onion gravy with dinner every night is receiving a cumulative dose. Allium damage adds up, and chronic low-level exposure can produce anaemia without any single dramatic event.
A packed cell volume and a blood smear looking for Heinz bodies. Treatment is supportive: oxygen if needed, intravenous fluids, and in severe anaemia a blood transfusion. There is no antidote — the body has to replace the damaged cells.