An inhibitor is a type of effector that decreases or prevents a chemical reaction. They are often called negative catalysts. Inhibitors are often used as medicines to decrease the rate of enzyme mediated reactions, but they can also act as poisons. Other examples include antioxidants and antipolymerization agents. A corrosion inhibitor is added to antifreeze (coolant) used in engines.

See also: receptor antagonist

Inhibitions in behavior may have a positive connotation (behaving properly, responsibly) or a negative one (being shy, boring, not brave). They are reduced by common forms of intoxication.