Chocolate contains an alkaloid called “theobromine”. Theobromine is in the same family as caffeine and is a type of stimulant (they both are mythylxanines). Theobromine stimulates the central nervous system, cardiovascular system, and causes a slightly increases blood pressure.
Dogs and certain other animals, such as horses and cats, cannot metabolize theobromine as quickly as humans can; this causes the above effects to be much more severe than is the case with humans. The specific notable side effects of toxic levels of theobromine in dogs includes: diarrhea; vomiting; increased urination; muscle twitching; excessive panting; hyperactive behavior; whining; dehydration; digestive problems; seizures; and rapid heart rate. Some of these symptoms, like the rapid heart rate, can ultimately be fatal to the dog.