Antidote: An agent that counteracts a poison and neutralizes its effects.

A chemical antidote is a substance that unites with a poison to form a harmless chemical compound. A mechanical antidote is a substance that prevents the absorption of a poison from the intestine into the body.

The word "antidote" is from the Greek antidotos which came from anti, against, + dotos, what is given = what is given against (something).