An antidote was used to counteract the effects of poisonous substances.
The milk of an Androzani Minor queen bat was an antidote for spectrox toxaemia. (TV: The Caves of Androzani)
The Seventh Doctor and Ace were able to bring a restorative to the Marat, allowing them to develop an antidote to the Gallifreyan virus being used against them by the Strykes. (COMIC: The Forgotten)
After the Tenth Doctor was poisoned by Arnold Golightly with cyanide, he improvised an antidote. It consisted of drinking ginger beer, eating walnuts for protein and anchovies for salt, and then getting a shock - a kiss from Donna Noble. This allowed him to eject the cyanide from his body through his mouth in a gaseous form. (TV: The Unicorn and the Wasp)
When forced to create EDTA to cure arsenic poisoning, Gwen Cooper used cyanide in the antidote. (TV: Rendition)
Angel Delight was a dessert that was an antidote for the poison in Hulzarrian carp. (PROSE: Five Dimensional Thinking)