Chocolate contains theobromine and theophylline, a naturally occurring stimulant in cocoa beans. This can cause vomiting, seizures, heart problems and even death for dogs.

After a dog has eaten a large quantity of chocolate, many dog owners assume their pet is unaffected. However, the signs of sickness may not be seen for several hours, with death following within twenty-four hours. A dog that has ingested a large quantity of chocolate will exhibit symptoms that include staggering, labored breathing, vomiting, diarrhea, abdominal pain, tremors, fever, heart rate increase, arrhythmia, seizures, coma or death.

Cocoa powder and cooking chocolate are the most toxic forms of chocolate to a dog. A 20 lbs dog can be seriously affected if it eats 3-5 oz. of unsweetened cooking chocolate. These forms of chocolate contain ten times more theobromine than milk chocolate. Thus, a chocolate mud cake could be a real health risk for a small dog. Even licking a substantial part of the chocolate icing from a cake can make a dog unwell.

Semi-sweet chocolate and dark chocolate are the next most dangerous forms, with milk chocolate being the least dangerous. A 20 lbs dog needs to eat more than a 6-9 oz. block of milk chocolate to be affected. Obviously, the smaller the dog, the less it needs to eat.