I'm sure most dog owners and non-dog owners know that you should never feed a dog chocolate. The oh so decadent chocolate is for humans only :)
Chocolate contains methylxanthines (found in cacao seeds) that causes diarrhea, panting, hyperactivity, tremors, abnormal heart rate, seizures, vomiting excessive thirst and urination.
There is a higher concentration of methylxanthines in dark chocolate than milk chocolate or white chocolate. Basically, the higher the cacao content the more methylxanthines.