Chocolate is toxic to dogs because it contains a chemical called theobromine, which is similar to caffeine. Theobromine is toxic to dogs because their bodies are not able to break it down effectively, which can cause them to become very sick. Symptoms of chocolate poisoning in dogs can include vomiting, diarrhea, rapid breathing, increased heart rate, and muscle tremors. In severe cases, it can even cause death. It's best to keep chocolate away from dogs to prevent them from getting sick.