antagonist
An antagonist is like a key that fits the lock but won't turn it — and while it sits there, the real key can't get in. It binds a receptor without activating it, so its main effect is to block the natural signal.
An antagonist has affinity but essentially zero intrinsic activity: it occupies the binding site without stabilizing the active state. A competitive antagonist binds the same orthosteric site as the agonist and can be overcome by enough agonist; a non-competitive antagonist binds elsewhere or binds so tightly that the agonist cannot displace it, capping the achievable response.
On its own, a pure antagonist does nothing in a system with no ongoing signaling — its effect appears only when there is an agonist (a drug or the body's own messenger) to block. Naloxone reverses opioid overdose by blocking opioid receptors; β-blockers like propranolol blunt adrenaline's drive on the heart.
Naloxone is an opioid receptor antagonist: given during an overdose, it binds the receptors and displaces the opioid, rapidly reversing respiratory depression.
An antagonist blocks the receptor without switching it on.
An antagonist that lowers a receptor's baseline activity below its resting level is not a plain antagonist but an inverse agonist — the distinction only matters for receptors with constitutive activity.