Hy`os*cy"a*mine (?), n. [See Hyoscyamus.] (Chem.) 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.