Chocolate made for human consumption can cause death in cats and dogs as they are sensitive to a class of chemicals called methylxanthines. Caffeine and theobromine are members of this family.
Our pets simply cannot metabolise and excrete methylxanthines as efficiently as humans. The half life of those compounds in the human body is in the order of two to three hours; in the dog it is more like 18 hours.
In a dog the compounds are taken up by the liver and transmitted via the bile into the intestine. They are then converted back into the original methyl-xanthines for another circuit through the animal. This repeats itself a number of times and instead of getting rid of the substances the animal will go through a cycle of poisoning itself.
There are many formulations of chocolate with varying amounts of caffeine and theobromine. The lethal dose of sweet milk chocolate for a dog is two ounces per kilogram of bodyweight. For a five kilogram dog this would be about 280 grams. A lethal dose of milk chocolate for a 25-kilogram dog would be about 1.4 kilograms. Dark chocolate is at least 10 times as lethal. A 25- kilogram dog could die from the methylxanthines in just five ounces.
Symptoms include vomiting, hyperactivity, restlessness, hyper-sensitivity to touch (a dog will jump when touched), very rapid heartbeat and an increased breathing rate, a loss of control of leg muscles, muscle tremor seizures, general weakness, coma and finally death.
In my opinion it would be a tragic mistake to encourage a dog to develop a taste for chocolate. A small dog left alone in a house with a box of chocolates might well follow its nose to the goodies and kill itself by poisoning, so be careful to keep any chocolate well out of reach.