以毒攻毒 (yǐdúgōngdú): fight poison with poison

First recorded in Tao Zongyi's Yuan-dynasty miscellany 《辍耕录》 (Chuō Gēng Lù), describing 骨咄犀 (gǔduōxī), a horn-like substance considered extremely poisonous yet able to neutralize other poisons: 其性至毒，而能解毒，盖以毒攻毒也 — 'its nature is utterly poisonous, yet it can cure poison; this is what is meant by fighting poison with poison.' Originally a TCM principle, now used metaphorically for any 'fight evil with evil' tactic.

usage

Often carries a hint of grim necessity — you reach for 以毒攻毒 when nicer methods have failed. Compare English 'fight fire with fire'.