The one thing I was mildly disappointed about here was that this wasn't a list of "drug and antidote" episodes but of drug episodes and antidote episodes...
When does the search of an antidote to a drug play a role in a plot? "City on the Edge" involves trying to give antidote to a drug, but there's no search. In the others, a cure to a disease or a natural poison is sought, not to a drug. Say, "Cloud Minders" and "Wink of an Eye" largely revolve around a classic antidote blackmail and intrigue scheme (c.f. Indiana Jones and the Temple of Doom), but the poison there is naturally occurring rather than something the villain would deliberately slip in the hero's drink.
The "artificial poison and antidote" plotline is among the most cliched ones, right alongside the amnesia plot and the evil twin plot. Probably we should be happy TOS never really dabbled in it.