Sad to say, chocolate can be poisonous to your dog. The offending substance appears to be theobromine, which can cause excitability, opisthotonus (head back spasm) and convulsions.
While sensitivity and susceptibility will inevitably vary from dog to dog, the only really safe advice is to say:
DON'T GIVE YOUR DOG CHOCOLATE!
This is an especially poignant warning around Easter time, when Easter eggs abound – and watch out for the canine thief, who might help himself to the whole stack! Be careful how and where you store Easter Eggs in the lead-up to Easter and warn children not to leave them lying around, not to feed them to dogs and not to let pieces fall.
If your dog does take some chocolate, seek prompt veterinary advice. It may be that timely use of homeopathic Nux vomica might reduce the harmful effects and buy more time.