Holly bush berries are poisonous to humans. They contain a chemical that act as emetics, inducing vomiting in individuals, especially children, who eat them. Fatalities from ingesting the berries are unknown, according to noted botanist Nancy Turner.
Authorities differ as to the cause of the nausea and vomiting that holly berries can cause. Some pin the blame on theobromine, an alkaloid found in smaller amounts in chocolate, and others on ilicin. The caffeine in the plant is also toxic to humans if they ingest it in large quantities. Interestingly, despite their toxicity to humans and some animals, the berries are an important source of food for rodents and birds.