Onion

Onion is a common kitchen ingredient, but it acts as a potent toxin for cats. Sulfur compounds like N-propyl disulfide oxidatively damage a cat's red blood cells and cause anemia, and the most dangerous route of exposure is sharing human food.

The three alliums

Onion belongs to the Allium genus along with garlic and chives. All three destroy red blood cells the same way, with garlic being the most concentrated. The nature of the risk is identical, so none of them should ever be given to a cat.

How cats get exposed

The key point: it's not only raw onion. Cooking it or grinding it into powder doesn't remove the toxicity, so soup broth, onion powder, and fried-food batter — anything with onion cooked in — are all dangerous. Don't leave chopped onion or peels on the counter, and toss onion scraps into a lidded bin so your cat can't dig them out.

How symptoms progress

Vomiting is usually the first thing you see. The dangerous part — hemolytic anemia — may not show up right away: lethargy, a racing heart, labored breathing, or reddish urine are signs that red blood cells are being destroyed. Blood in the urine is the clue you are most likely to catch in the litter box, so glance at the color when you scoop. Because these signs can be delayed, a cat often looks fine at the time of exposure.

What to do right now

If you suspect ingestion, don't wait even if your cat looks fine — go to the vet. Anemia shows up late, so the fact that exposure happened is itself a key diagnostic clue to share with the vet.