Chocolate is bad for dogs primarily because it contains two compounds: caffeine and theobromine. Both are stimulants that humans can metabolize easily, but dogs process them much more slowly. This slow metabolism causes these substances to accumulate in a dog's system, overstimulating their central nervous system and heart. The result can be symptoms ranging from vomiting, diarrhea, increased thirst, and restlessness to more severe outcomes like muscle tremors, seizures, cardiac arrhythmias, coma, or even death. The darker and more bitter the chocolate, the higher the amount of these toxic compounds, making dark chocolate and unsweetened baking chocolate particularly dangerous for dogs. Even milk chocolate can be harmful depending on the quantity consumed relative to the dog's size. White chocolate contains very little theobromine but is still not safe due to fat and sugar content. In summary, chocolate's toxicity in dogs is due to the theobromine and caffeine it contains, which dogs cannot effectively metabolize, leading to potentially serious or fatal poisoning.