A very common misconception is that chocolate in and of itself can kill a dog. This, however, is not true. Chocolate contains theobromine which in high doses will cause heart arrhythmia, seizures and death to an animal who does not have the enzymes to metabolize it. The amount of chocolate needed for this reaction varies by size and weight, but typically ranges from 10-100 grams per kilogram (2.2 pounds) of body weight. A small dog would need about 100 g/kg or 230 g (1 ounce) while a large one would need closer to 500g/kg or 1100 g (4 ounces).
While the actual fatal dose varies by dog, any chocolate can be harmful. Generally speaking, milk chocolate is less dangerous than dark or baking chocolate. Semi-sweet baking chips are worse for dogs than semi-sweet eating chips which contain slightly more baking chocolate and slightly less sugar. White chocolate contains only cocoa butter and has little to no theobromine so it is not dangerous. That said, eating any type of chocolate can cause vomiting and diarrhea in most dogs.
A more common issue with chocolate is that it contains caffeine which stimulates the central nervous system. This can cause restlessness, heart arrhythmia, muscle twitching and hyperactivity in some cases. For this reason it is best to keep all chocolate away from pets.
Though this myth has been debunked, Prevention Magazine still recommends against feeding your dog chocolate because of the caffeine and other potentially dangerous ingredients.