Onions are toxic to cats; your cat should never eat them, even in tiny portions or as a seasoning. Onions contain a compound called thiosulfate, which can cause a condition called hemolytic anemia in felines. This is a potentially life-threatening illness where red blood cells get destroyed faster than they're produced. Unlike humans, cats (and dogs!) lack the enzymes required to break down the thiosulfate present in onions.
Other foods from the allium family of plants — including garlic, leeks, shallots, and scallions — are also extremely unsafe for cats.
Even small amounts of onion can have serious consequences for your furry friend, with symptoms including lethargy, vomiting, diarrhea, panting and discolored urine. If left untreated, onion toxicity could lead to more severe complications like liver damage, organ failure, or death.
Onions are found in a lot of human foods, so you need to be careful about which foods you're feeding to your cat as treats. Even foods that are safe when served plain will instantly become unsafe if they're topped or seasoned with onions. And onion powder and dehydrated onions are just as dangerous as other types of onions.
If you know your cat has eaten onions, get them to the vet right away, even if they're asymptomatic. Your vet may need to induce vomiting or administer other treatments. The sooner your cat is treated, the better.
If your cat requires veterinary treatment due to eating onions (or any other unsafe food), pet insurance can help cover the cost of treatment.