At least some content in this article is derived from information featured in the latest update of Pottermore. As such, spoilers will be present within the article. Please take care when reading this article if you have not yet been through the latest update.
The use of mistletoe berries as an ingredient in the Antidote to Common Poisons is ironic: the non-berry parts of the mistletoe plant are extremely poisonous, causing intense stomach pain, diarrhea, low pulse, visual disturbances, and convulsions if eaten. However, the berries are considered "relatively non-toxic" if only a small amount are ingested, but can cause stomach upset when eaten in larger quantities.