Why onions and garlic are toxic to dogs
All plants in the Allium family contain sulfur compounds that are released when the plant is chopped, chewed, cooked or digested. In dogs these compounds damage red blood cells: small clumps called Heinz bodies form inside the cells, the cells break apart, and the dog becomes anemic. Severe cases also change hemoglobin into a form that cannot carry oxygen.
Cooking does not destroy the toxin. Dogs have been poisoned by raw, cooked, dehydrated and granulated onion and garlic. Dried products such as onion powder, garlic powder, onion flakes and dry soup mixes pack a lot of plant into a small amount.
How the calculator works
The Merck Veterinary Manual reports that 15 to 30 g of raw onion per kg of body weight has caused clinical signs in dogs, and that garlic is 3 to 5 times more toxic than onion. The calculator converts what your dog ate into grams, divides by its weight in kilograms and compares the result with that range. Garlic is counted as 3 to 5 times onion.
onion dose (g/kg) = grams eaten ÷ dog's weight in kg
garlic: same, then × 3 to × 5
Example: a 20 lb (9.1 kg) dog eats half a medium onion, about 55 g. 55 ÷ 9.1 ≈ 6 g/kg, well below the 15 g/kg level. If the same dog eats 55 g of garlic (about 18 cloves), the onion-equivalent dose is 18 to 30 g/kg, which is in the toxic range.
Toxic amounts by dog weight
Fresh onion and fresh garlic that reach the level where signs were reported (15 g/kg of onion). For garlic the table uses the stronger end (5 × onion), so it shows the cautious figure.
| Dog's weight | Onion | Garlic |
|---|---|---|
| 5 lb (2.3 kg) | 34 g (⅓ medium onion) | 7 g (2 cloves) |
| 10 lb (4.5 kg) | 68 g (⅔ medium onion) | 14 g (4–5 cloves) |
| 20 lb (9 kg) | 136 g (1¼ medium onions) | 27 g (9 cloves) |
| 40 lb (18 kg) | 272 g (2½ medium onions) | 54 g (18 cloves) |
| 60 lb (27 kg) | 408 g (3¾ medium onions) | 82 g (27 cloves) |
| 80 lb (36 kg) | 544 g (5 medium onions) | 109 g (36 cloves) |
Medium onion 110 g, garlic clove 3 g (USDA FoodData Central). These are the amounts at which signs were reported, not safe limits; dried and powdered products reach them with far less.
Signs of onion or garlic poisoning
An upset stomach can come within a day, but the main problem, anemia, takes time. Heinz bodies start to form within 24 hours and peak around 3 days; red blood cells usually break down 3 to 5 days after the dog ate the onion or garlic.
- Weakness, lethargy, tiring quickly on walks
- Loss of appetite
- Pale or yellowish gums
- Fast breathing and a fast heartbeat
- Red-brown urine
- Collapse in severe cases
What to do if your dog ate onion or garlic
- Remove what is left and keep the packaging or recipe.
- Estimate the amount with the calculator and note when your dog ate it.
- Call your vet or a poison hotline, especially for a large amount, a small dog, or dried or powdered onion or garlic.
- Do not induce vomiting at home unless a vet tells you to.
- Watch your dog for the next 5 days even if it seems fine, and call the vet at the first sign of weakness or pale gums.
If the meal was recent, a vet may make your dog vomit. Later treatment focuses on the anemia: checking the red blood cell count, fluids to protect the kidneys, and oxygen or a blood transfusion in severe cases.
Frequently asked questions
Can dogs eat onions?
No. Onions contain sulfur compounds that damage a dog's red blood cells and can cause anemia. Raw, cooked, fried and dried onion are all toxic, and onion powder is more concentrated than fresh onion.
How much onion is toxic to a dog?
According to the Merck Veterinary Manual, 15 to 30 g of raw onion per kg of body weight has caused signs in dogs. For a 20 lb (9 kg) dog that is about 136 to 272 g, or roughly one and a quarter to two and a half medium onions. Smaller amounts can still matter for small or sick dogs, so call your vet.
Is garlic worse than onion for dogs?
Yes. The Merck Veterinary Manual describes garlic as 3 to 5 times more toxic than onion. At the cautious end, a 20 lb (9 kg) dog could reach the level where signs were reported with about 27 g of fresh garlic, or about 9 cloves.
How long after eating onion will a dog show signs?
Damage to red blood cells starts within 24 hours, but the anemia usually becomes obvious 3 to 5 days later. Watch for weakness, tiring quickly, pale gums, fast breathing and red-brown urine, and call your vet even if your dog seems fine at first.
My dog ate food cooked with onion. Is that dangerous?
Cooking does not make onion safe. A small amount of onion spread through a large meal is usually far below the toxic range, but gravy, soup mixes and dishes with onion powder can be concentrated. Use the calculator above with your best estimate and call your vet if you are unsure.
Sources
- Merck Veterinary Manual: Garlic and Onion (Allium spp) Toxicosis in Animals, by Cristine Hayes, DVM, ASPCA Animal Poison Control Center (reviewed September 2024): toxic dose in dogs, relative potency of garlic, timing of anemia, signs and treatment.
- USDA FoodData Central: Onions, raw and Garlic, raw (portion weights).
- ASPCA Animal Poison Control Center and Pet Poison Helpline (contact numbers, checked October 2026).
This calculator is for general information only and does not replace a veterinarian. Dogs react differently, products vary, and a low estimate does not mean your dog is safe. If your dog ate onion or garlic and you are unsure, contact a vet or a pet poison hotline. Last updated: October 2026.