Chocolate contains a chemical (theobromine) that poisons dogs. http://www.dogownersdigest.com/news/library/chocolate-dog-poisoning.shtml

I've heard it is. Not sure

Hope this helps. DOGS AND CHOCOLATE Danger Give chocolate to loved ones, and you could end up poisoning them. That is, if the loved one is your pet dog. Even small amounts of Theo bromine, an ingredient in chocolate, can cause vomiting and restlessness in pets. Larger doses can be fatal. While most pet owners expect a dog to develop an upset stomach after eating a large amount of chocolate, few realize its danger or toxic potential. The lethal dose of Theo bromine depends on the size of the dog and the type of chocolate. Ounce for ounce, baking chocolate has six to nine times as much of the substance as milk chocolate does. Estimates of the smallest amounts that can be fatal are: 4 ounces of milk chocolate or 1/2 ounce of baking chocolate for small dogs, such as Chihuahuas and toy poodles. 8 ounces of milk chocolate or 2 ounces of baking chocolate for medium-sized dogs, like cocker spaniels and dachshunds. 8 ounces of milk chocolate or 4 ounces of baking chocolate for large dogs, including collies and Labrador retrievers. Cats have much different eating habits and seldom are poisoned by chocolate. While a very small amount of chocolate may not harm some dogs, it's safest to avoid giving it to them at all. If an accident occurs, a veterinarian should be consulted. Treatment may require inducing vomiting, stabilizing the animal's heartbeat and respiration, controlling seizures and slowing the absorption of Theo bromine. If the animal already is comatose, its stomach may need to be pumped. Theo bromine does not completely leave the dogs system, as do some foods, my research informs me that Theo Bromine has a half life of 17.5 hours. This means that half of it has gone in 17.5 hours; in 35 hours another half of the half that was remaining will have gone. We can go on and on forever and ever getting less and less, to be silly about it, it never gets to zero! There always has to be something left for there to be a half of something. Its like a radio wave/signal travelling into space, it will reduce by half over a given distance, it repeats this time and again, but it never gets to zero. Trying to find it after a time will be difficult then imposssible of course, as with Theo bromine. So we have a time factor to consider also apart from a safe amount. The Theo bromine in other sweets and biscuits, cakes, coated in chocolate or with chocolate chips etc. will be less of course, but its wise to avoid all these things. Health food shops stock, none chocolate choc drops for humans; I am told these are fine, but, its good practise to ask for the ingredient list to check for Theo bromine and then ask your VET. And remember ingredients can change without notice. As in all case of animal safety, I cant say this enough, ask your vet, this page is no exception, don’t take my word as being accurate. All dogs can be different, as are humans; it may be there is a dog out there that could be killed by the smallest dose of chocolate. This page does not suggest that any dose is safe, on all matters covered here; please ask your vet who is your correct authority. http://www.transportcafe.co.uk/dogchocolate.html

Chocolate contains theobromine. A naturally occurring stimulant found in the cocoa bean, theobromine increases urination and affects the central nervous system as well as heart muscle.

I took this from the APSCA site: Chocolate - Did you know…chocolate can be harmful or even deadly to pets? Depending on the form involved, chocolate can contain high amounts of fat and caffeine-like substances known as methylxanthines. If ingested in significant amounts, chocolate can potentially produce clinical effects ranging from vomiting and diarrhea to panting, excessive thirst and urination, hyperactivity, abnormal heart rhythm, tremors, seizures and even death in severe cases. Typically, the darker the chocolate, the higher the potential for clinical problems from methylxanthine poisoning. White chocolate has the lowest methylxanthine content, while baking chocolate contains the highest. As little as 20 ounces of milk chocolate, or only two ounces of baking chocolate can cause serious problems in a 10-pound dog. While white chocolate may not have the same potential as darker forms to cause a methylxanthine poisoning, the high fat content of lighter chocolates could still lead to vomiting and diarrhea, as well as the possible development of life-threatening pancreatitis, an inflammatory condition of the pancreas. They have a complete list of things you should safeguard from your pets, including other candies and plants that are toxic. Here's their link for your future reference: http://www.aspca.org/site/PageServer?pagename=pro_apcc_dyk#4

Some say it is bad because the chemicals in chocolate are toxic for dogs. I say it depends on your dog. My dog is 11 and he loves chocolate. Now I dont feed it to him like it is going out of style but it doesnt hurt him. Heck the world oldest dog (recently past away) had eaten a whole bag of Reese's peanut butter cups and lived to be 21. That was a dachshund. So if you have a dog and you are tempted to give him chocolate start with a little and see if it has any affects. Here is the story about the worlds oldest dog, Chanel. http://news.yahoo.com/s/ap/us_obit_world_s_oldest_dog

In short: Yes. It can be deadly.