1) Act in opposition to 2) Antagonise 3) Antagonize 4) Counterbalance 5) Deactivate 6) Defuse 7) Frustrate by contrary action 8) French word used in English 9) Make safe 10) Move 11) Offset 12) Reduce the effect 13) To act in opposition to 14) Work against 15) Work in opposition to 1) Antagonise 2) Counterbalance 3) Countervail 4) Derail 5) Negate 6) Neutralize 7) Undermine 8) Undo 
• (v. t.) 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.
Coun`ter·act' transitive verb [ imperfect & past participle Counteracted ; present participle & verbal noun Counteracting .] To act in opposition to; to hinder, defeat, or frustrate, by contrary agency or influence; as, to counteract
[v] - oppose and mitigate the effects of by contrary actions
counteract 1. To oppose and to mitigate the effects of something by contrary action; to check. 2. To prevent something from having an effect, or to lessen its effect.
neutralize 2 counterbalance verb oppose and mitigate the effects of by contrary actions; `This will counteract the foolish actions of my colleagues`
to act in opposition to; frustrate by contrary action.