Chocolate naturally contains theobromine, a stimulant in the caffeine family. Toxic levels vary according to chocolate concentration and the size of dog but it's best to keep all chocolate away from your dog. Signs that your dog has eaten chocolate may include hyper excitability, hyper irritability, increased heart rate, restlessness, increased urination, muscle tremors, vomiting and diarrhea. If you think your dog has eaten chocolate, call your veterinarian immediately.