The Collaborative International Dictionary of English v.0.48:

Antispasmodic \An`ti*spas*mod"ic\, a. (Med.)
   Good against spasms. -- n. A medicine which prevents or
   allays spasms or convulsions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antispasmodic
    n 1: a drug used to relieve or prevent spasms (especially of the
         smooth muscles) [syn: antispasmodic, spasmolytic,
         antispasmodic agent]