For dogs, a chemical in chocolate called theobromine is the source of the problem.
Theobromine is similar to caffeine. Theobromine is toxic to a dog when it ingests between 100 and 150 milligrams per kilogram of body weight.
Different types of chocolate contain different amounts of theobromine: It would take 20 ounces of milk chocolate to kill a 20-pound dog, but only 2 ounces of baker’s chocolate or 6 ounces of semisweet chocolate. It is not that hard for a dog to get into something like an Easter basket full of chocolate eggs and bunnies and gobble up a pound or two of chocolate. If the dog is small, that could be deadly.
It turns out that chocolate poisoning is actually not as unusual as it sounds. For a human being, caffeine is toxic at levels of 150 milligrams per kilogram of body weight. That’s the same as for dogs! Humans generally weigh a lot more than dogs, but small children can get into trouble with caffeine or chocolate if they consume too much of it.