No really... if you're a dog, I am telling you now to NOT EAT CHOCOLATES. They can kill you... literally!
I was reading at askipedia.com and here's the reason:
Unfortunately for dogs chocolate tastes really, really good, but chocolate is
not really poisonous, not like cyanide. That doesn't change the fact that
it can kill your dog. It is theobromine that is harmful to dogs, if
you can find chocolate without it you'll have the happiest dog in town.
Theobromine is a stimulant found in the coca plant. People often get
theobromine confused with caffeine, while they have similar effects, eating a
piece of chocolate will not give a human the rush a cup of coffee does.
Chocolate will still give a sugar rush, but don't eat it if you're up late
studying for an exam.
Dogs and horses are greatly affected by
theobromine. Chocolate is a banned substance in horse racing because of
the boost it will give to the horse. Of course a horse can still eat too
much of it, like a dog, but they're significantly bigger animals and it would
take much more chocolate.
More at this site.