Chocolate can be lethal for cats and dogs, the reason being they are unable to metabolise a substance in chocolate called Theobromine. The effects of theobromine include increase heart rate, increased central nervous activity and dilation of arteries. It causes vomiting, diarrhoea, increased urination, excitability, convulsions and can result in cardiac failure and death. If your pet has accidentally eaten chocolate, give it medicinal charcoal and take it immediately to the vet.