what is it in Chocolate that is bad for dogs?

It’s the Theobromine.

I asked the same thing:
http://www.fluther.com/disc/3437/why-is-chocolate-poisonous-to-dogs/

It’s because dogs can’t digest it properly.

Here is some data that was recently on my Greyhound Rescue group:

How much chocolate is to much ? It is important to know just how much chocolate can produce toxic effects on your dog and that amount is one hundred to one hundred and fifty miligrams.That measurement to most does not mean much so to simplify it if you have a twenty pound dog that means to reach the level of becoming poisoned they have to eat approximately ten ounces depending on th type of chocolate. Bakers chocolate and unsweetened chocolate are harsher types for any dog to eat only because the chocolate is purer therefore they contain a higher dosage of Theobromine than the regular candy bar type chocolate.

If you suspect your animal has eaten chocolate what signs do you look for? Toxic signs occur quickly so even if there is any question if your animal has consumed chocolate or not, take the animal to the vet. Do not wait to see if the animal will be alright because any delay in treatment can be devastating. Your dog, if untreated, can quickly go into convulsions, have arrythmia, stop breathing and go into a coma.

First your dog may start vomiting on its own. If not you can induce vomiting with a hydrogen peroxide solution of half peroxide and half water.

The animal will also experience severe diarreah which can cause the animal to dehydrate quickly.

Seek medical help immediately. Be sure to take to the vet the unfinished chocolate if any is left, and any wrappings from the chocolate. This will help the vet to identify the type of chocolate and the concentration of the candy.

The vet will begin treatment and induce vomiting if vomiting has not already occured. Usually the vet will use activated charcoal to induce. The vet will also start an IV for hydration and also to administer medication. The vet will also administer anti-seizure medication because animals that have ingested chocolate have a ninety eight percent chance of having severe seizures. If your animal is having extreme symptoms or cardiac problems he will easily be able to give those cardiac medications through the IV. The stay after this treatment in an easy case is usually two days, although in more extreme instances the hospital stay for your animal could be weeks.