An antidote was a substance used to counteract the effects of poison or toxin.
In the year 2266, Leonard McCoy learned that the antidote to the plague that had stricken Argylus was genitum lars. (TOS comic: "Child's Play")
In 2267, Leonard McCoy provided the antidote for Akwood's syndrome to the Giant Brain. (TOS comic: "The Brain-Damaged Planet")