Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

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