Avocados can cause stomach upset in dogs, and their large round seeds represent a choking hazard. Avocados contain a toxin called persin that is lethal to birds and some herbivores but has little effect on dogs besides mild gastrointestinal distress.
At worst, persin causes vomiting and diarrhea in dogs that eat large quantities of avocado flesh or peel. In other animals, the same chemical causes cardiac problems and a host of other symptoms, including death. Birds are especially susceptible to the toxin, while dogs, cats and humans are mostly immune. The greatest danger avocados present to dogs is intestinal blockage, as the large seeds are indigestible if swallowed.