Onion toxicity chart by dog weight
The table shows how much fresh onion reaches the commonly used threshold of 0.5% of body weight in onion-equivalent. The Merck Veterinary Manual reports clinical signs in dogs at 15-30 g/kg of onion, so the threshold below is a cautious level for calling your vet, not a "safe" amount.
| Dog weight | Fresh onion at threshold | Roughly | As powder |
|---|---|---|---|
| 5 lb (2.3 kg) | 11.3 g | 10% of a medium onion | 1.4 g (≈ 0.6 tsp) |
| 10 lb (4.5 kg) | 22.7 g | 21% of a medium onion | 2.8 g (≈ 1.2 tsp) |
| 20 lb (9.1 kg) | 45.4 g | 41% of a medium onion | 5.7 g (≈ 2.4 tsp) |
| 40 lb (18.1 kg) | 90.7 g | 82% of a medium onion | 11.3 g (≈ 4.7 tsp) |
| 60 lb (27.2 kg) | 136.1 g | 124% of a medium onion | 17 g (≈ 7.1 tsp) |
| 80 lb (36.3 kg) | 181.4 g | 165% of a medium onion | 22.7 g (≈ 9.4 tsp) |
Powder is about 8 times stronger than fresh by weight, because the water has been removed. Repeated small amounts add up over several days.
Why onion is toxic to dogs
Onions contain organosulfur compounds (such as n-propyl disulfide) that oxidize hemoglobin in red blood cells. Damaged cells form Heinz bodies and are destroyed, causing hemolytic anemia. Cooking does not destroy the toxins. Japanese breeds such as Akitas and Shiba Inus are reported to be more sensitive.
Symptoms and when they appear
Stomach upset (vomiting, diarrhea, drooling) can start within a day. Signs of anemia usually appear 1-5 days later: weakness, tiredness, pale gums, fast breathing or heart rate, and red or brown urine.
Foods that often contain onion
- Gravy, stock and bouillon
- Soups, stews and casseroles
- Pizza, burgers and meatloaf
- Stuffing and onion rings
- Some baby foods (often fed to sick pets) and seasoned meats
What to do
- Estimate how much was eaten and in what form, and use the calculator.
- Call your vet or poison control: ASPCA Animal Poison Control: (888) 426-4435 · Pet Poison Helpline: (855) 764-7661. If it was recent, your vet may induce vomiting.
- Watch for weakness, pale gums and dark urine over the next 5 days, and recheck with your vet if you see them.
Also see: Can dogs eat onions? · How much garlic is toxic?
Frequently asked questions
How much onion is toxic to a dog?
Many vets treat about 0.5% of body weight in onion (5 g per kg) as concerning; Merck reports clinical signs at 15-30 g/kg. For a 20 lb dog, 0.5% is about 45 g, less than half a medium onion.
Is cooked onion safe for dogs?
No. Cooking does not destroy the toxic compounds. Onion powder is even more concentrated.
My dog ate a small piece of onion. Is that okay?
A single small piece is unlikely to harm a medium or large dog, but call your vet for advice, especially for small dogs, Japanese breeds or repeated exposure.
How long does onion toxicity last in dogs?
Anemia usually develops over 1-5 days and recovers over a couple of weeks once exposure stops. Severe cases may need a blood transfusion.