How much chocolate is harmful to dogs depends on the type of chocolate as well as the health and size of your dog. Theobromine is the toxin in dogs that makes it harmful. The average candy bar or milk chocolate has about 60 mg/oz. This means that if your dog weighs about 20 pounds then it would take 1 pound of chocolate to be poisonous. On the other hand, baking chocolate and semi-sweet chocolate contains much more theobromine which means only a few squares could make your dog sick.