In "The Borgias", Season 3, Episode 1, the second Borgia pope, Rodrigo Borgia or Pope Alexander VI, is cured of poisoning by the forced ingestion of activated charcoal.
The first attestation of the use of activated charcoal that I could find was 1834. ("Activated Charcoal--Past, Present and Future", Drs Derlet and Albertson. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1306980/?page=1)
Does anyone know of an earlier reference?
Edit: for the correct episode
I know I've encountered charcoal and clay as poison antidotes in classical medicine...
This corporate site claims that Galen wrote "almost 500 treatises on the use of charcoal in medicine." I'm not sure I trust that source, but this article discussing "the medical applications of activated carbon in the treatment of intoxication and poisoning caused by external (exogenous) events" (behind a paywall, sorry) confirms that Galen used charcoal to treat poison.