An Anti-intoxicant is a chemical agent that is used to either reverse or prevent the effects of alcohol intoxication.
While enroute to Megara, Deanna Troi accepted a drink from former 20th century businessman Ralph Offenhouse, being unaware that the drink contained actual alcohol rather than synthale. As a result she became quite drunk, and was taken to sickbay where she was given an anti-intoxicant to reverse the effects of the drinks she ingested. (TNG novel: Debtors' Planet)
Later, a Starfleet team that had infiltrating a ceremony inducting new members into the Klingon Order of the Bat'leth were given anti-intoxicants prior to leaving on their mission. Prior to the ceremony the three humans on the team - Benjamin Sisko, Odo, and Miles O'Brien had consumed so much blood wine that if not for the anti-intoxicant Worf believed that they would not have been able to continue standing. (DS9 episode: "Apocalypse Rising")