Source : Webster's Revised Unabridged Dictionary (1913)

Counteractive \Coun`ter*act"ive\ (-?kt"?v), a.
   Tending to counteract.

Counteractive \Coun`ter*act"ive\, n.
   One who, or that which, counteracts.

Source : WordNet®

counteractive
     adj : opposing or neutralizing or mitigating an effect by contrary
           action [syn: {antagonistic}] [ant: {active}]