Simon Whistler, host of the informative video series “Today I Found Out” explains why the specific alkaloid theobromine makes chocolate so potentially lethal to pets, particularly dogs.
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. …Cats also are particularly susceptible to poisoning from chocolate for the same reason dogs are. However, unlike dogs, cats generally aren’t particularly inclined to eat chocolate, having no “sweet” taste receptors.