Larger amounts of chocolate, particularly of the most toxic type, can bring about epileptic seizures in some dogs, and in all dogs, can kill.Poisoning of dogs by chocolate is not as uncommon as you might think.
"Chocolate ingestions are one common reason why pet owners and veterinarians call us," said Dana Farbman, Certified Veterinary Technician and Manager, Client and Professional Relations, ASPCA Animal Poison Control Center. "However, it would be difficult to verify an exact ranking in frequency of calls, as the types of substances we receive calls on can vary greatly depending on many factors, including the time of year. We generally do experience somewhat of a rise in chocolate calls around holidays, such as Halloween, Easter, Christmas, Valentine's Day and Mother's Day.
Chocolate contains theobromine. A naturally occurring stimulant found in the cocoa bean, theobromine increases urination and affects the central nervous system as well as heart muscle. While amounts vary by type of chocolate, it's the theobromine that is poisonous to dogs.