But there is a long history of some species of the plant being used as an herbal supplement - treating epileptic seizures to cancer and viral infections. Herbal supplements are not regulated or tested by the FDA, and studies on the effects of the plant have not been conclusive, but European mistletoe (Viscum album) is used by some to combat illness.
Don't confuse this with the mistletoe above the doorway at Christmas! That's American mistletoe and it's toxic, particularly to pets. Ingestion of the North American variety of the plant has been shown to lead to death within 10 hours because of reduced heart rate, gastrointestinal distress, and narcotic effects.