·noun An alkaloid found in henbane (Hyoscyamus niger), and regarded as its active principle. It is also found with other alkaloids in the thorn apple and deadly nightshade. It is extracted as a white crystalline substance, with a sharp, offensive taste. Hyoscyamine is isomeric with atropine, is very poisonous, and is used as a medicine for neuralgia, like belladonna. Called also hyoscyamia, duboisine, ·etc.