A counteragent was a type of drug that negated the effects of an illness or other drug.
Makara herbs were a counteragent for the sedatives Doctor Julian Bashir prescribed to Kira Nerys during her pregnancy in 2372. (DS9: "The Darkness and the Light")
The Doctor described the elusive, as-yet-unfound cure to Chakotay and Janeway's unknown disease as a counteragent. (VOY: "Resolutions")