Avocados are considered dangerous to dogs because they contain a toxin called persin. And even more unfortunately, the variety of avocado most toxic to dogs — the Guatemalan — is the most common variety found in US supermarkets. Hence the continued veterinary cautiousness about administering anything containing traces of avocado to dogs. Avocados can cause severe gastrointestinal irritation in canines, including vomiting and diarrhea.