Chocolate contains a xanthine compound, theobromine, that is toxic in sufficient quantities. Examples of other xanthine compounds are caffeine and theophylline. The toxicity from all of these compounds is similar.

It takes a fairly large amount of chocolate to cause problems. In "The Handbook of Small Animal Practice" by Dr. Rhea Morgan, the following data is given: The toxic dose of theobromine is about 100 to 150mg/kg Milk chocolate contains 6mg of theobromine per ounce. Semi-sweet chocolate contains about 22mg/oz. and baking chocolate about 35 to 45mg/oz.

From this it is pretty easy to see that milk chocolate poses only a minor threat while the other forms of chocolate can be a bigger problem.

Xanthines affect primarily the central nervous system, the cardiovascular system and peripheral nerves.