While rarely fatal, chocolate ingestion can result in significant illness for dogs and cats. Chocolate is toxic because it contains a chemical called theobromine, as well as caffeine. Theobromine is the main toxin in chocolate and is very similar to caffeine. Dogs cannot metabolize theobromine and caffeine as well as people can, which makes them die.

The amount of toxic theobromine varies with the type of chocolate. The darker and more bitter the chocolate, the more dangerous it is to dogs. Dark chocolate are highly concentrated and contain 130~450 mg of theobromine per ounce. Common milk chocolate only contains about 44~58 mg/ounce. Even if the amount ingested is not a toxicity concern, dogs can still become ill from the fat and sugar in chocolate, because they have more sensitive stomachs. 

Image Citation: Stregowski, J. (2021, July 16). How Much Chocolate Is Toxic to Dogs? The Spruce Pets. https://www.thesprucepets.com/how-much-chocolate-is-toxic-to-dogs-4706542

Clinical signs depend on the amount and type of chocolate ingested. Complications such as developing aspiration pneumonia from vomiting, can make the prognosis for chocolate poisoning worse. 

If your dogs eat chocolate accidentally, you can buy a treatment, and it depends on the amount and type of chocolate eaten. If treated early, decontamination including vomiting and administering activated charcoal to prevent absorption of theobromine into the body. Treatments of activated charcoal may be repeated to reduce the continued resorption also.

Reference: Chocolate Poisoning in Dogs | VCA Animal Hospital | VCA Animal Hospitals. (n.d.). Vca. https://vcahospitals.com/know-your-pet/chocolate-poisoning-in-dogs