The Collaborative International Dictionary of English v.0.48:

Succedaneum \Suc`ce*da"ne*um\, n.; pl. Succedanea. [NL. See
   Succedaneous.]
   One who, or that which, succeeds to the place of another;
   that which is used for something else; a substitute;
   specifically (Med.), a remedy used as a substitute for
   another.
   [1913 Webster]

         In lieu of me, you will have a very charming
         succedaneum, Lady Harriet Stanhope.      --Walpole.
   [1913 Webster]

WordNet (r) 3.0 (2006):

succedaneum
    n 1: (medicine) something that can be used as a substitute
         (especially any medicine that may be taken in place of
         another)