The answer depends on the type of chocolate, how much was eaten, and the size of your dog.

Chocolate contains methylxanthines (caffeine and theobromine) which humans can break down into non-toxic substances quickly, but dogs (or cats) cannot. These methylxanthines cause diarrhea, vomiting, and restlessness and can progress to seizures and heart failure resulting in death. Only one square (1 oz) of unsweetened baker’s chocolate could kill a 6 lb Chihuahua, but would likely result in no symptoms in a 70 lb Labrador. Less cocoa means less toxin so milk chocolate is less deadly – a 6 lb Chihuahua eating 1 oz of milk chocolate (about 6 Hershey’s KISSES) might result in only mild symptoms such as diarrhea or vomiting. The 70 lb Labrador would need to eat about 63 Hershey’s KISSES to have the same symptoms. Although cats are less likely to eat candy they too can suffer the same toxic effect from eating chocolate. Keep all chocolate out of reach of your pets. Now might be a good time to teach your dog “LEAVE IT”,. If they do eat chocolate call your Vet or ASPCA Poison Control (888-426-4435) immediately. They will need to know your pet’s weight, the amount and the type of chocolate eaten. They may recommend giving your dog hydrogen peroxide to induce vomiting. Hydrogen peroxide must be fresh to be effective (keep an unopened bottle on hand for this purpose) and SHOULD NOT be used in cats. Your pet may need emergency treatment at an animal hospital. The faster an animal is treated the more likely they will recover.