Has your dog ever eaten chocolate? Of all candies, chocolate poses the biggest threat to dogs. The darker and more bitter the chocolate, the more poisonous it is. Methylxanthines are the chemicals in chocolate that are dangerous to pets, and they are more concentrated in darker chocolates. If you think your dog may have ingested chocolate, signs to watch for include vomiting, diarrhea, lethargy, agitation, increased thirst, an elevated heart rate, and in severe cases, seizures.