Antagonist.

A molecule that blocks a receptor without activating it.

An antagonist binds a receptor without activating it, occupying the site so that agonists cannot produce their effect. Antagonists can be competitive, meaning enough agonist can out-compete them for the binding site, or non-competitive, meaning they bind elsewhere on the receptor or block the signal downstream and cannot simply be overwhelmed by adding more agonist. In the peptide world antagonists appear less often in consumer conversation than agonists do, but they matter for understanding how these systems are studied. Researchers frequently confirm that a peptide works through a specific receptor by showing its effect disappears when a known antagonist for that receptor is present. Antagonists also appear as therapeutics in their own right, for example in the hormone axis where blocking a receptor is the goal rather than stimulating it. There is a related idea worth keeping separate: an inverse agonist does more than block, it actively pushes a receptor below its baseline level of activity, which is not the same thing as an antagonist doing nothing on its own. The common misreading is treating antagonist as a synonym for harmful or for side effect. It is a mechanism description, and whether blocking a given receptor is desirable depends entirely on the context.