Onions belong to the Allium family, which also includes garlic, leeks, and chives. They contain sulfur compounds that form sulfur-containing oxidants when chewed, cooked, or processed. These oxidize hemoglobin inside red blood cells, forming damaged clusters called Heinz bodies and triggering methemoglobin formation; the body then destroys these damaged red blood cells, which can lead to hemolytic anemia. The Merck Veterinary Manual identifies cats as the most susceptible species to this effect, more so than dogs.
Merck reports that toxicosis has been documented in cats after ingesting less than a teaspoon of cooked onion, or around 5 g/kg of raw onion — substantially less than the roughly 15–30 g/kg reported to cause signs in dogs. Raw, cooked, dehydrated, and powdered onion are all considered toxic, and effects can be cumulative from repeated small exposures, so treat any onion exposure as a genuine risk rather than a minor one.
Yes — the Merck Veterinary Manual identifies cats as the most susceptible species to onion toxicity, developing signs from smaller amounts (roughly a teaspoon of cooked onion, or about 5 g/kg raw) than the doses reported to affect dogs.
Contact a veterinarian or pet poison hotline right away, noting how much and what form it was in. Signs of anemia can be delayed, so do not wait for symptoms.
No. Onion powder is concentrated and is toxic to cats, and some baby foods, soups, and sauces contain it, so check the label before sharing.
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General educational guidance based on the references above, not veterinary advice. See our editorial policy for how we choose sources.
This tool is intended for educational purposes only and should not be used as a substitute for emergency veterinary advice, diagnosis, or treatment.
If your pet may have consumed toxic foods, chocolate, medications, chemicals, or other dangerous substances, seek immediate help from a licensed veterinarian or emergency animal service in your area. Do not delay professional care based on calculator or search results alone.
Pet poison hotlines (24/7, US and Canada):
ASPCA Animal Poison Control Center: (888) 426-4435
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Pet Poison Helpline: (855) 764-7661
Pet Poison Helpline states its service covers the U.S., Canada and the Caribbean. The ASPCA line is also used by Canadian pet owners. A consultation fee may apply to both. In Canada, your nearest 24-hour emergency vet clinic is the best first call; elsewhere, use local emergency veterinary directories.
How we calculate results: This tool uses commonly referenced veterinary toxicity thresholds and food safety guidelines. Actual toxicity can vary based on your pet's size, health status, and exact substance consumed. These results are estimates only and do not replace a veterinary assessment.