Chocolate is made of cocoa beans, and cocoa beans contain theobromine and caffeinea, which are toxic to dogs.
Q: How much chocolate is too much?
Dogs react differently to toxins. Reactions may vary according to size, age and overall health of the dog. Theobromine is present in various amounts depending on the kind of chocolate, with white chocolate containing the least and baking chocolate/cocoa powder the most.
Q: What are the warning signs of a problem?
Watch your dog carefully for symptoms ranging from vomiting and diarrhea to panting, excessive thirst and urination, and for signs of pancreatitis.
Q: What can be done once chocolate has been ingested?
Make note of the type and amount of chocolate consumed, if possible. Within two hours of consumption, try inducing vomiting unless your dog is over stimulated, comatose or has lost his gag reflex. If your dog has eaten a significant amount of chocolate, or displays any of the above-mentioned symptoms, contact your vet or emergency clinic immediately.
(Information taken from Dr. Carolyn Schweitzer alldogs.info)