Blog/Nutrition

Are onion and garlic toxic to dogs and cats?

Updated August 2, 2026·4 min

ONION AND GARLIC · CATS AND DOGS

Neither cats nor dogs should have them, and cats have it worse: the Merck veterinary manual calls cats the most susceptible species, with dogs next in line. A cat needs 5 g of raw onion per kilogram of body weight, or less than a teaspoon of cooked onion; a dog needs 15–30 g/kg. Garlic is 3–5 times more toxic than onion. Nothing shows up straight away: red blood cells usually break down on days 3–5.

01How much onion is dangerous

Species and food

Threshold in the manual

Scaled to a weight

Cat, onion

5 g/kg raw, or under a teaspoon cooked

4 kg cat — about 20 g

Dog, onion

15–30 g/kg raw

10 kg dog — 150–300 g

Cat, garlic

3–5 times more toxic than onion

4 kg cat — 4–7 g

Dog, garlic

3–5 times more toxic than onion

10 kg dog — 30–100 g

The third column is arithmetic off the threshold: the manual sets a dose per kilogram, and the weight is ours to plug in. It is not a portion anyone is allowed. For your own animal, multiply the threshold by its weight. The gap between species stays that wide: what already counts for a 4 kg cat has not even started the count for a 10 kg dog.

02The powder beats the fresh bulb

Poisonings are tied to concentrates more often than to fresh onion. A fresh bulb in the bowl is visible; dried flakes and powder are not, and they arrive with the leftovers of dinner. Cooking neutralises nothing: the feline threshold in the manual is set for cooked onion in the first place.

The whole genus is dangerous: onion, leek, shallot, garlic, chives.
Concentrates here means dried flakes, onion and garlic powder, packet soups and seasonings.
Boiled, fried and roasted onion stays dangerous: heat does not help here.
Garlic is 3–5 times stronger than onion, so a pinch of it is not a harmless measure.

That also answers the folk advice to give a dog garlic against worms: in the manual garlic sits among the causes of poisoning, and it is 3–5 times more toxic than onion. No safe dose is given for it. A deworming schedule is the vet’s call. The rest of what should stay off the plate is collected in foods toxic to dogs.

03Why a quiet first day proves nothing

The mechanism is sulphur-containing oxidants that damage red blood cells. First come Heinz bodies: marks of that damage. They start forming within the first 24 hours and peak at around 72. The breakdown of the red cells itself (haemolysis) usually lands on days 3–5 after the meal.

“He ate it yesterday and he is fine” is therefore not an argument. By the time an owner has decided nothing happened, the count has barely reached day two. What the decision runs on is species, weight and how much went in.

04What to do if it has already been eaten

Call your own clinic or the nearest 24-hour one: give the species, the weight and what exactly was eaten.
In the US you can also ring Pet Poison Helpline on 855-764-7661 or the ASPCA poison control centre on 888-426-4435.
Keep the seasoning packet or soup sachet: the vet works out the dose per kilogram from the ingredients.
Never make a cat vomit at home: hydrogen peroxide causes severe bleeding inflammation of the gullet and stomach.
Salt, ipecac syrup and fingers down the throat are banned in both species: that is a second poisoning on top of the first.

Feline emetics are prescription drugs given by a vet, and even for those the manual quotes an effectiveness that drops to 50%. In dogs, vomiting at home is allowed only where veterinary care cannot be reached: 3% hydrogen peroxide at 1–2 ml per kilogram, never above 45 ml, and never at all if the animal is dull, swallows badly or is unsteady. What else does not suit a cat from ordinary human food is in milk and treats.

Platera keeps your pet’s weight to hand

These thresholds are counted per kilogram, and cats and dogs differ several times over. It is the first figure a clinic asks for.

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05Frequently asked

Can I give my dog garlic for worms?

No. Garlic is 3–5 times more toxic than onion, the mechanism is the same damage to red blood cells, and the veterinary manual gives no safe dose for it. For a 10 kg dog the scaled threshold already works out at roughly 30–100 g of garlic. A deworming schedule belongs to your vet.

My cat ate a bit of onion from the soup — is that dangerous?

It needs counting. The manual gives two separate thresholds for cats: under a teaspoon of cooked onion, or 5 g of raw onion per kilogram of body weight. One is not a conversion of the other: cooked is counted in spoons, raw in grams per kilogram. Call your clinic or the nearest 24-hour one and give the cat’s weight and roughly how much it ate. Do not induce vomiting at home in a cat.

How long does onion poisoning take to show?

Not on day one. Heinz bodies (marks of damage to red blood cells) form within the first 24 hours and peak at around 72. The breakdown of the red cells itself usually falls on days 3–5 after the meal. A quiet first day therefore guarantees nothing.

Is cooked or fried onion safer than raw?

No, heat does not neutralise it. If anything the reverse: the feline threshold in the manual is set for cooked onion, at under a teaspoon. And pets are poisoned by concentrates more often than by a fresh bulb: dried flakes, onion and garlic powder, packet soups and seasonings.

Are leeks and shallots dangerous too?

Yes, the whole genus is: onion, leek, shallot, garlic and chives. The lists of foods to avoid for dogs and for cats agree on this point in the veterinary leaflets. There is no safe member inside the genus. On strength the manual compares only garlic with onion (garlic is 3–5 times more toxic), and gives no separate thresholds for leek or shallot.

Sources

Merck Veterinary Manual — Food Hazards↗

WSAVA Global Nutrition Guidelines and owner toolkit↗

ASPCA Animal Poison Control — People Foods to Avoid Feeding Your Pets↗