The Collaborative International Dictionary of English v.0.48:

Synergist \Syn"er*gist\, n. [Cf. F. synergiste.]
   1. One who holds the doctrine of synergism.
      [1913 Webster]

   2. (Med.) A remedy which has an action similar to that of
      another remedy, and hence increases the efficiency of that
      remedy when combined with it.
      [1913 Webster]

WordNet (r) 3.0 (2006):

synergist
    n 1: a drug that augments the activity of another drug [ant:
         antagonist]