I heard that you should never give your dog chocolate except for special 'dog chocolate'. What effect does normal chocolate have on a dog?

From what I've heerd and readed, the bromine in chokolate kills dawgs.

The first symptoms are digestive upset, vomiting and diarrhea. I think it progresses to liver failure and death. Fortunately it takes a large dose to harm most dogs. It is not bromine, but a bromine containing compound that causes the damage.

There are carob containing products that are safe for dogs. However, dogs will do just fine without them, and I see no reason for developing a taste for a chocolate like foods.

It is called theobromine. The more pure the chocolate, the higher the amount of theobromine. Dark chocolate contains more of this than milk chocolate. Theobromine affects the central nervous system and cardiovascular system. It is a compound that is in the same family as caffeine. About 100 mg of theobromine is deadly for a dog. Aside from the symptoms labman has mentioned, epileptic seizures, hyperactivity, increased urination, muscle twitching, panting can also occur. Usually the symptoms start within a few hours of ingestion. To get an idea of how bad it is for a dog, 2 ounces of bakers chocolate is highly toxic to a 15 pound dog.

As labman states, it is better not to allow them to develop a taste for any kind of chocolate. THAT is the main problem that vets encounter. When a dog develops a taste for carob or the doggie treats that taste like chocolate, there is an increased chance that they will get into it if you accidentally leave any on the counters. There are enough safe doggy treats that are made today, that there isn't any need to give them those treats that mimic people food.

One of the dogs at my dogpark had recently had surgery. Her owners left a glass baking dish of brownies on the counter to cool, and she knocked it down and ate half the pan including the broken glass. That was expensive emergency surgery, because they couldn't just make the dog vomit up the chocolate because of the glass.

So yes, it's best if your dog never gets a taste for chocolate in the first place.