Caffeine for one but the worst is a chemical called 'theobromine' which is highly toxic to all animals and present in higher amounts than even caffeine. And it is present in ALL chocolate period. It may be less concentrated in milk chocolate because of other ingredients but its present in all chocolate and if you love your fog dont feed it ANY chocolate and risk a deadly dose.
Theobromine is bad for dog and is present in black chocolate white chocolate is good for dogs.Onion and garlic can kill a dog in two days.Other harm. : grapes, raisins, avocado, yogurt, yeast or baking, caffeine, ham.