What is the term for a substance that acts against the action of another substance in the body?

The term that describes a substance that acts against the action of another substance in the body is "antagonist." In pharmacology and physiology, antagonists bind to receptors or enzymes and inhibit their activity, counteracting the effects of agonists, which are substances that activate these receptors or enzymes.

For example, in the context of drugs, an antagonist might block the effects of a neurotransmitter, preventing the associated physiological response that would normally occur. This property makes antagonists crucial in various medical treatments, especially when it comes to managing conditions that require modulation of receptor activity.

In contrast, terms like "agonist" relate to substances that mimic or enhance the action of other substances. "Inhibitor" generally refers to a substance that decreases the function of an enzyme or receptor but doesn't necessarily imply the opposing action characteristic of an antagonist. An "antidote" specifically refers to a substance that counteracts the effects of a poison or toxin rather than opposing another physiological action.