toxic for them

"In addition to a high fat content, chocolate contains caffeine and theobromine, two different types of stimulants that affect the central nervous system and the heart muscle, as well as increasing the frequency of urination."

http://tinyurl.com/6olnz5

chocolate contains theobromine that is toxic to dogs in sufficient quantities. This is a xanthine compound in the same family of caffeine, and theophylline.

Chocolate contains a natural occurring stimulant called theobromine found in the cocoa bean plant Theobroma cocoa, the bean that makes chocolate. Theobromine is the poison as it affects the central nervous system,..

as well as the heart of the dog, throwing their system into panic which often manifests in the form of epileptic seizures.

http://www.bigpawsonly.com/chocolate-poisoning-in-dogs-health.htm