inverse agonist
Some receptors are slightly 'on' even with no signal present, like a tap that drips on its own. An inverse agonist turns that idling activity down below its resting level — not just blocking the signal, but actively quieting the receptor further.
This is possible because many receptors spontaneously flip into their active shape some of the time, giving a low baseline (constitutive) signal. An inverse agonist preferentially binds and stabilizes the inactive shape, shifting the balance and reducing the basal output. A neutral antagonist, by contrast, simply holds the receptor's baseline where it already is.
The distinction only matters when a receptor has meaningful constitutive activity; in systems with little or none, an inverse agonist and a neutral antagonist behave indistinguishably. Many drugs once labelled antagonists — certain antihistamines and some β-blockers — turn out to be inverse agonists when tested against receptors that signal at rest.
Many 'antihistamines' such as cetirizine are inverse agonists at the H1 receptor: they not only block histamine but lower the receptor's constitutive signaling below baseline.
An inverse agonist drives a constitutively active receptor below its resting baseline.
Inverse agonist, neutral antagonist, partial agonist, and full agonist form a continuous spectrum of efficacy — from strongly negative, through zero, to strongly positive.