Sprig of wolfsbane
Eating a sprig of wolfsbane will cure lycanthropy.[1]
Wolfsbane is a vegan food, and is also suitable food for herbivorous pets.
Wielding wolfsbane has no effect on lycanthropes or wolves (as opposed to garlic or other banes). You can't use wolfsbane to poison weapons, and it will not poison you if you eat it.
Eating a cursed sprig of wolfsbane might interrupt your meal, so that half of the sprig remains uneaten. If you finish the meal it will nevertheless grant you relief from lycanthropy.
Generation
Priests begin the game with 1 or 2 sprigs of wolfsbane.
Origin
NetHack is not real life; in real life, wolfsbane is extremely poisonous and should not be eaten. The name comes from the belief that it was unusually lethal to wolves.[2]
Encyclopedia entry
1. Any of various, usually poisonous perennial herbs of the genus Aconitum, having tuberous roots, palmately lobed leaves, blue or white flowers with large hoodlike upper sepals, and an aggregate of follicles. 2. The dried leaves and roots of some of these plants, which yield a poisonous alkaloid that was formerly used medicinally. In both senses also called monkshood.