An antitoxin was an antibody with the ability to neutralize a specific toxin.
In 2268, Leonard McCoy told James T. Kirk there was no antitoxin for poison of the mugato after Kirk had been bitten by the creature. (TOS: "A Private Little War")
Ryetalyn was the only known antidote for Rigelian fever, and as such an antitoxin could be made from it. However this antitoxin could be rendered inert and useless by irillium. (TOS: "Requiem for Methuselah")
In 2269, McCoy administered the antitoxin dylovene to Hikaru Sulu when he was poisoned by a retlaw plant. (TAS: "The Infinite Vulcan")