Why is chocolate poisonous to dogs? How much can they eat without becoming affected?

Cacao contains a component called theobromine which is poisonous to many animals. 

The amount of theobromine varies between different products but the darker the chocolate is, the more poisonous it is. There is ca 5-15mg of theobromine per gram dark chocolate, while there is ca 2mg of theobromine per gram light chocolate (such as milk chocolate). White chocolate contains a lot less theobromine. Ca 20mg theobromine per kg body weight is required to poison a dog. 

Symptoms of poisoning are generally seen ca 2-4 hours after consumption. It can, however, take up to 24 hours until any signs of poisoning are seen. The symptoms include restlessness, nausea, vomiting, drooling and diarrhoea. Heart palpitations, instability and seizures are also seen in some cases. 

If your dog has eaten chocolate you should seek emergency veterinary care immediately so that vomiting can be induced. Your dog may also need to be hospitalised depending on its state and the amount and type of chocolate it has eaten.