Cocoa and dark chocolate generally contain more theobromine and caffeine than lighter chocolate. A small amount of one product can therefore mean a completely different risk from the same amount of another. Signs of poisoning can include vomiting, restlessness, panting, a high heart rate, trembling and, in severe cases, seizures or effects on the heart.
Try to find out your dog's approximate weight, which chocolate product it ate, the cocoa content if it is stated, how much is missing and when it happened. Keep the packaging if you can. This information makes it easier for the vet to estimate the exposure and decide whether treatment needs to start before symptoms develop.
Contact a vet as soon as possible if your dog has eaten a potentially harmful amount. Do not wait for symptoms before you call. Do not make your dog vomit and do not give home remedies without instructions from a vet.