Dogs cannot eat chocolate because it contains two substances, theobromine and caffeine, which are toxic to them. Dogs metabolize these chemicals much more slowly than humans, allowing toxic levels to build up in their bloodstream. Theobromine, in particular, has a stimulating effect on the central nervous system and heart, and can cause symptoms ranging from vomiting and diarrhea to serious issues such as increased heart rate, tremors, seizures, and in severe cases, even death. The darker the chocolate, the higher its theobromine content and thus the greater the risk to dogs. Even small amounts of dark chocolate or cocoa powder can be very harmful, especially to smaller dogs.