Chocolate—Confectionary chocolate and dogs do not mix. While chocolate is not poisonous, theobromine which is found in chocolate is. Theobromine triggers epileptic seizures in susceptible animals, and can cause cardiac irregularity leading to myocardial infarct and death. Additionally, chocolate irritates the gastrointestinal tract, even to the extent that it causes such internal bleeding that it can kill within a couple of days.
Induce vomiting; give activated charcoal slurry. http://www.workingdogs.com/doc0175.htm