According to Pet Poison Helpline, holly is poisonous to dogs and cats. Varieties of holly, such as English, Chinese and Japanese holly, all contain saponins, which can harm animals when ingested. Holly also contains other toxic substances, such as cyanogens and methylxanthines.
According to Pet Education, holly berries are also toxic to animals when consumed. Animals that consume holly leaves or berries typically experience symptoms such as drooling, vomiting and diarrhea. Head shaking is another symptom of holly consumption; however, head shaking is typically the result of injury from the leaves. Animals that ingest holly plants should be promptly treated by a veterinarian.