True. 
Years ago, my wife and I took a leisurely stroll with our beloved dog, Cody. The walk was uneventful until Cody came to a dead stop that nearly pulled my arm out of its socket. Since he wasn’t doing his doggy duty, I closely inspected my pet to learn the reason for his odd behavior. I discovered that he had found a perfectly intact chocolate bon bon on the ground, and was holding it gently in his mouth, head down, indicating that he knew full well that it was forbidden. To make a long story short, my wife was panicked, certain that Cody would die on the spot and Cody was just as desperate to keep his prize. Amazingly, the dog actually forfeited the chocolate after much coaxing, and lived to see another day. While one bon bon may not have actually killed Cody, it is true that chocolate is harmful to dogs in large quantities, and could result in serious health problems or possibly even death. The authors of a short article which appeared in the British Medical Journal (2005) wrote “The potential hazards to humans of eating too much chocolate are well known (obesity and dental caries to name but two), but you may be unaware that chocolate is potentially lethal to dogs”. There is a chemical in chocolate called theobromine, and this is what is so harmful to dogs. Different types of chocolates contain different amounts of theobromine, however. White chocolate contains very little theobromine and is usually not considered to be harmful to canines. Milk chocolate contains 60 mg per oz, semisweet chocolate 160 mg per oz, and bakers chocolate about 450 mg per oz. Consuming theobromine will likely stimulate a dog’s central nervous system and heart as well as increase their blood pressure. Additional negative effects could include vomiting, diarrhea, muscle spasms, excessive panting, and increased urination. If you have a dog that consumes chocolate, the best thing to do would be to call your veterinarian immediately.
Reference:
Finlay F & Guiton S: Chocolate Poisoning. British Medical Journal (2005), Vol 331, pg 633