Why is chocolate harmful?

Please excuse my stupidity. I have read many times that feeding my Maltese chocolate is harmful. I could never find out why. Is there an ingredient in chocolate that causes it to be so dangerous? Its not harmful for humans. I need and would like to know why this is so.
Estelle

• I'll try my best to answer your concerns about the dangers of chocolates and your Maltese. First, don't feel *stupid* as it often surprises many pet owners to find that chocolate is harmful and can be poisonous if sufficient amounts are consumed.

  The culprits in chocolates is caffeine and theobromine(??spelling), which both are of the drug class of methylxanthines that cause chocolate to be toxic to your maltese. Not much needs to be consumed by any animal really but typically a dog to prove fatal. A good rule of thumb would be 1/2 oz of baking choc. to a pound of body weight. So assuming your Maltese weighs say.. 6 lbs, ONLY 3 ounces would be poisonous.

  If your Maltese would ever accidentally ingest chocolate, which is not a rare occurance, you should try to find out how much he/she consumed and call your veterinarian with the details so he can determine if there should be any need for alarm.

  SOME of the signs that occur from a dog consuming chocolate include vomiting, excessive urination, hyperactivity, fast breathing, weakness and seizures. Death can occur usually due to the reaction on the heart from the drugs mentioned above. Hope this helps.
  Jay