The Collaborative International Dictionary of English v.0.48:

Counteract \Coun`ter*act"\ (koun`t?r-?kt"), v. t. [imp. & p. p.
   Counteracted; p. pr. & vb. n. Counteracting.]
   To act in opposition to; to hinder, defeat, or frustrate, by
   contrary agency or influence; as, to counteract the effect of
   medicines; to counteract good advice.
   [1913 Webster]