Chocolate is not inherently poisonous to dogs. The danger has more to do with the quantity they ingest and they type of chocolate. There are 2 compounds in chocolate that are cause for concern, caffeine and theobromine. These are stimulants, and dogs are more sensitive to these compounds than humans are.
Two other factors are important. The type of chocolate and the fact dogs will gorge and eat as much as possible in a short amount of time. Milk chocolate has the lowest levels of caffeine and theobromine. Dark chocolate, baking chocolate, and cocoa powder have increasing amounts of those two stimulants. This means if a 50 pound dog eats 2 ounces of milk chocolate, there is little cause for concern. However, if the same dog eats 2 ounces of cocoa powder, it would need immediate emergency care and could die if not treated.
What are the signs of chocolate toxicity?
In mild cases we may see vomiting diarrhea and some trembling. More severe cases can cause very fast heart rate, abnormal heart rhythms, trembling, panting, restlessness, seizures, liver and kidney damage, and even death.
What should you do if your dogs eats chocolate?
If it has happened within the past hour, get your dog to the vet. Often we induce vomiting to get rid of any undigested chocolate.
If it has been more than an hour, your dog may need to hospitalized, monitored, and treated for any signs of chocolate toxicity.
Always, and I mean always, bring the wrapper/packaging with you if possible. Also, try to determine how many pieces your dog ate. If your dog happens to have eaten a chocolate cake, it is helpful to know what kind of chocolate and how much was used in the recipe.
WebMd has a handy little chocolate toxicity calculator. This can help you determine how concerned you should be about the amount of chocolate your dog ingested.
The ASPCA also has an animal poison control center. You can call them, and they can help you determine if you need to be concerned about the amount of chocolate your dog ingested. There is a fee for this service. Call 888-426-4435