Allium cepa
Why is it risky?
Onions contain N-propyl disulfide and other organosulfur compounds that cause oxidative damage to feline red blood cells, leading to Heinz body hemolytic anemia. Cats are more sensitive than dogs due to differences in hemoglobin structure. Onion powder is far more concentrated, and therefore more dangerous per gram, than fresh onion.
Affected Systems
Digestive · Cardiovascular
Symptoms
Early: vomiting, diarrhea, drooling. Delayed (2–5 days): pale or yellowish gums, weakness, lethargy, rapid or labored breathing, reduced appetite, reddish-brown urine, collapse.
What To Do
Vet visit for any significant ingestion. Blood work to assess red blood cell damage. Severe anemia may require blood transfusion. Do not induce vomiting at home.
Notes
All Allium family members are toxic to cats: garlic (approximately 5× more potent than onion per gram), chives, leeks, and shallots. Many processed human foods contain onion or garlic powder, always check ingredient labels on broths, stocks, baby food, and commercial treats.
Sources
→ ASPCA Animal Poison Control Center — aspca.org/pet-care/animal-poison-control/toxic-and-non-toxic-plants/onion
→ Merck Veterinary Manual — https://www.merckvetmanual.com/toxicology/food-hazards/garlic-and-onion-allium-spp-toxicosis-in-animals?query=onion