Counteractant is a word that refers to an agent or substance that opposes the effects of another substance or agent. Some synonyms for the term include neutralizer, antidote, inhibitor, and antagonist. A neutralizer is a substance that neutralizes the effects of another substance or agent. An antidote is a substance that counteracts the effects of a poison or toxin. An inhibitor is a substance that prevents or slows down a chemical reaction. An antagonist is a substance that opposes the action of another substance or agent. These synonyms for counteractant are useful in various fields, including medicine, chemistry, and biology.