Cats are obligate carnivores with a metabolism that handles some everyday foods very badly. Onions, garlic, chives, and leeks damage red blood cells and can cause anemia, even cooked or powdered.
Chocolate and anything with caffeine contain compounds cats can't metabolize, affecting the heart and nervous system. Grapes and raisins have been linked to kidney failure in cats and dogs alike, even in small amounts.
Alcohol and raw yeast dough are dangerous too โ dough can expand in the stomach and ferment into alcohol as it rises. Xylitol, a sweetener in sugar-free gum and some peanut butters, causes a rapid, dangerous drop in blood sugar.
When in doubt, keep human food off the menu and stick to cat-specific treats โ and call your vet or the ASPCA Poison Control line right away if you suspect your cat has eaten something risky.