The thiosulfates in onions — along with related Allium plants like garlic, leeks and chives — damage red blood cells and cause Heinz body hemolytic anemia. Raw, cooked and powdered forms are all dangerous, and cats are more sensitive than dogs. Garlic is roughly 3-5 times more toxic than onion by weight. Broths, sautéed dishes and other foods flavored with onion can also pose a risk, so keep pets away from seasoned human food.