Remedy to counteract the effects of a poison or toxin. Administered by mouth, intravenously, or sometimes on the skin, it may work by directly neutralizing the poison; causing an opposite effect in the body; binding to the poison to prevent its absorption, inactivate it, or keep it from fitting a receptor at its site of action; or binding to a receptor to prevent the poison's binding there, blocking its action. Some poisons are not active until converted to a different form in the body; their antidotes interrupt that conversion.