If a dog consumes a large amount of a toxic type of chocolate, the result can be a racing heart rate, heart arrhythmia, tremors, seizures or even death. The type and amount consumed determines the dog's reaction.
The general rule about chocolate and dogs is that the more bitter and the darker the chocolate, the more dangerous it is. An occasional chocolate chip cookie does not harm a dog, but a full square of baker's chocolate or dark chocolate may have harmful effects on any dog's health. The size of the dog must also be considered when deciding whether he consumed a dangerous amount as larger dogs can usually tolerate larger amounts of chocolate.