Chocolate can be lethal for cats. Although most cats won't eat it on their own, they can be coaxed to eat it by owners and others who think they are giving the cat a treat. The toxic agent in chocolate is theobromine. It's in all kinds of chocolate, even white chocolate. The most dangerous kinds, though, are dark chocolate and unsweetened baking chocolate. Eating chocolate can cause abnormal heart rhythm, tremors, seizures, and death.