The Collaborative International Dictionary of English v.0.48:

Hyoscyamine \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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Duboisine \Du*bois"ine\, n. (Med.)
   An alkaloid obtained from the leaves of an Australian tree
   (Duboisia myoporoides), and regarded as identical with
   hyoscyamine. It produces dilation of the pupil of the eye.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hyoscyamine
    n 1: a poisonous crystalline alkaloid (isometric with atropine
         but more potent); used to treat excess motility of the
         gastrointestinal tract