Chocolate contains theobromine (a compound similar to caffeine) which is poisonous to dogs. A dose of 50 mg/lb can be fatal to a dog. Milk chocolate contains 45 mg of theobromine per ounce and unsweetened baking chocolate contains 400 mg per ounce.
Just one ounce of unsweetened baking chocolate can kill a small breed dog. Theobromine when ingested by dogs causes release of epinephrine (adrenaline) which causes the heart to race and serious cardiac arrhythmias to develop.
Signs of chocolate poisoning in dogs include vomiting, diarrhea, excessive urination, hyperactivity followed by depression and coma, seizures, and death.
Emergency care for a dog ingesting chocolate includes inducing vomiting and administration of charcoal to reduce absorption of the chocolate remaining in the stomach and intestines. Drugs to stabilize the heart are needed as well as intravenous fluids and drugs to prevent shock. Mortality is high and care for these cases is intensive and expensive.
Owners should be prudent to prevent dogs from finding and consuming chocolate. Theobromine is also present in cocoa beans, cola, and tea.
If you suspect your dog has ingested chocolate, call your veterinarian immediately. With their instruction you may be able to induce vomiting greatly increasing the odds of your pet surviving. Time is of the essence in these cases - seek emergency care immediately if chocolate has been consumed. Keep chocolate and other products containing theobromine out of reach from your dogs.
Be sure to instruct children and visitors to not feed you dog chocolate as a "treat".