Holly belongs to the plant genus Ilex. 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 if consumed in large amounts, may result in gastrointestinal irritation and depression of the central nervous system. True poisonings are relatively uncommon, since the plant is not very palatable. However, due to possible side effects, it’s still advisable to keep holly and other Ilex plants out of your pooch’s reach.