The Collaborative International Dictionary of English v.0.48:

Therapeutic \Ther`a*peu"tic\, Therapeutical \Ther`a*peu"tic*al\,
   a. [F. th['e]rapeutique, Gr. ?, from ? attendant, servant, ?
   to serve, take care of, treat medically, ? attendant,
   servant.] (Med.)
   Of or pertaining to the healing art; concerned in discovering
   and applying remedies for diseases; curative. "Therapeutic or
   curative physic." --Sir T. Browne.
   [1913 Webster]

         Medicine is justly distributed into "prophylactic," or
         the art of preserving health, and therapeutic, or the
         art of restoring it.                     --I. Watts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

therapeutical
    adj 1: relating to or involved in therapy; "therapeutic approach
           to criminality" [syn: therapeutic, therapeutical]