Chocolate is toxic to dogs because it contains a substance called theobromine. Theobromine is a methylxanthine alkaloid that is found in cocoa beans. It is similar to caffeine, but it is more toxic to dogs. Theobromine can cause a variety of symptoms in dogs, including:

* Vomiting

* Diarrhea

* Increased thirst

* Increased urination

* Panting

* Rapid heartbeat

* Seizures

* Death

The amount of chocolate that is toxic to a dog depends on the size of the dog and the type of chocolate. Dark chocolate is more toxic than milk chocolate, and white chocolate is not toxic at all. The following table lists the amount of chocolate that is toxic to a 10-pound dog:

* Dark chocolate: 1 ounce

* Milk chocolate: 4 ounces

* White chocolate: not toxic

If you think your dog has eaten chocolate, it is important to take them to the vet immediately. The vet can assess your dog's condition and provide treatment if necessary.