Chocolate can kill your dog. How? Chocolate contains theobromine, which is toxic. Theobromine not only is a stimulant to the central nervous system as well as the cardiovascular system, it increases blood pressure and can cause nausea and vomiting.
Baker's chocolate (390-450mg/oz) has 10 times the amount of theobromine as milk chocolate (44-60mg/oz). White chocolate has very little theobromine, so if your dog has woofed down some Easter candy, let's hope it was white chocolate.
How much does it take to be fatal to your dog? Depends on your dog's weight. For every 2 pounds, 100 mg is toxic. So, if your dog weighs 10 lbs., then 500 mgs is fatal - or one ounce of Baker's chocolate.
Ref: The Merck Veterinary Manual, 8th ed.
