Can it be true? Is chocolate poisonous to dogs? The answer is yes. The hazard to your dog however, depends on the type of chocolate, the size of dog, and the amount consumed.
The component of chocolate that is toxic to dogs is called theobromine. Whereas humans easily metabolise theobromine, dogs process it much more slowly allowing it to build up to toxic levels in their system.
A large dog can consume a great deal more chocolate than a small dog before it will suffer ill effects. It's also worth remembering that different types of chocolate have different levels of theobromine. Cocoa, cooking chocolate and dark chocolate have the highest levels while milk chocolate and white chocolate have the lowest.
A small amount of chocolate will probably just give your dog an upset stomach. He may throw up or have diarrhoea. Large amounts though, will have a more serious effect. In sufficient quantities, theobromine can produce muscle tremors, seizures, an irregular heartbeat, internal bleeding, or a heart-attack.
What to look out for
The onset of theobromine poisoning is usually marked by severe hyperactivity.
Don't worry if your dog has eaten a single chocolate or helped himself to the last square of your bar, because this won't provide a large enough dosage of theobromine to hurt him. If you have a small dog though, and he has eaten a box of chocolates, you need to get him to the vet immediately. And if you're dealing with any quantity of dark or bitter chocolate, err on the side of caution. The high level of theobromine in dark chocolate means it takes only a very small amount to poison a dog; just 25 grams may be enough to poison a 20 kg dog.
The usual treatment for theobromine poisoning is to induce vomiting within two hours of ingestion. If you are worried that your dog may have eaten a large quantity of chocolate do not hesitate to call your vet. Time will be of the essence.