Holly berries are poisonous to both humans and pets. A child who consumes just a couple of holly berries may not experience any problems, but prolonged vomiting, nausea and diarrhea can occur when more than three berries are consumed, according to the Children's Hospital of Philadelphia.
Holly is an evergreen shrub that is often allowed to grow into a tree. The bark, seeds and leaves of the holly shrub are poisonous and contain high levels of theobromine, which is a caffeine-like alkaloid. Curiously, the same compound is found in chocolate, albeit in much lower concentrations than in the holly shrub.