What makes chocolate so lethal to dogs is two things...Theobromine and Caffeine. The higher the amount of Theobromine the more toxic it is to the dog. According to the American Kennel Club, the type of chocolate most dangerous to dogs is Cocoa powder followed by unsweetened baker's chocolate, dark chocolate, and milk chocolate.
The other factors that affect the outcome of your dog getting into chocolate include the amount the dog has eaten and the size of the dog with smaller dogs needing a smaller amount than bigger dogs in order to have adverse reactions. Dogs who are older or have heart conditions are also at greater risk when they ingest chocolate.
Since accidents do happen and dogs will still find a way to get to the chocolate at times what should you do in such a situation? Gather whatever remaining evidence you can find including the wrapper left behind and how much you think the dog ate and head right for the vet with the information which will help them determine the severity of the situation.
Other than heading for the vet immediately two important numbers to have on hand are the Pet Poison Helpline (855-213-6680) or the ASPCA Animal Poison Control Center (888-426-4435) for help.