Unfortunately, Travis, winterberry belongs to the plant genus Ilex, of which most holly species are a part. Plants of this genus contain caffeine-like substances and saponins, as well as a bitter-tasting protein known as ilicin. All parts of the plant are considered potentially toxic, and may result in gastrointestinal irritation and central nervous system depression if very large amounts are consumed.
Because the plant is not very palatable, we’ve found that true poisonings are relatively uncommon. However, due to the possible effects we can see from ingestions, we do advise keeping winterberry and other Ilex plants out of the reach of pets.