Allium vegetables contain sulfur compounds that oxidize hemoglobin in red blood cells, and feline hemoglobin is especially vulnerable to this kind of damage. The injured cells are destroyed faster than the body can replace them — hemolytic anemia. Gram for gram, garlic is several times more potent than onion, so "just a little garlic" is not the safer option it sounds like.
The illness is delayed: oxidative damage starts within a day, but weakness, pale gums, fast breathing, and lethargy typically appear several days after the exposure, which makes the cause easy to miss. For cats the more common route is not a slice of raw onion but human food that contains allium as an ingredient — broth, gravy, seasoned meat, onion or garlic powder. It is worth checking labels before sharing any savory human food with a cat.
Sources
- Garlic and Onion (Allium spp) Toxicosis in Animals — Merck Veterinary Manual
- People Foods to Avoid Feeding Your Pets — ASPCA Animal Poison Control
This page is informational and is not veterinary advice. If your pet has eaten something risky, contact your vet or an animal poison control line right away.