Dogs that eat chocolate can suffer from seizures, diarrhea, vomiting and in severe cases, death, as noted by WebMD. The chemical in chocolate, especially dark chocolate and baker's chocolate, is theobromine. A dog's digestive system does not process theobromine the way a human's does, leading to a toxicity buildup that can be fatal.
Chocolates with the highest concentration of cocoa also have higher concentrations of theobromine. Milk and white chocolate are less likely to cause a fatal reaction in dogs than dark chocolate.
Dogs that ingest large quantities of chocolate should receive veterinary care to help them regurgitate the chocolate. A veterinarian can administer medications that will absorb the remaining amount of theobromine and recommend what to do if a dog is vomiting or suffering from gastrointestinal distress after eating chocolate.