Chocolate can be very poisonous to your cat. There is a chemical in chocolate called Theobromine. Theobromine is a diuretic as well as a cardiac stimulant. This can cause the pet's heart rate to increase or it may cause the heart to beat irregularly, both of which can be dangerous to the animal. The level of Theobromine present in chocolate varies depending on the type of chocolate. For example, dark chocolate contains higher levels than milk chocolate and so could pose a greater risk to your pet. In order of toxicity: Dark Chocolate (Most Toxic)>Baking Chocolate>Semisweet Chocolate>Milk Chocolate>Hot Chocolate>White Chocolate (Least Toxic). The ASPCA animal poison control will give you more specific information than your local poison control as they specialize with animal poisoning, however they do have a minimal charge.