One usually uses the term "inhibitor" to refer to a substance that reduces the catalytic activity of an enzyme, and "antagonist" to refer to a substance that blocks binding of the natural ligand to a receptor and thereby prevents signaling by the receptor.

An irreversible antagonist binds covalently and cannot be displaced by either competing ligands or washing. Inhibitors are drugs that can bind to a protein, such as an enzyme and decrease its activity.