Antagonist

An antagonist is a molecule that binds a receptor without activating it, blocking the site so the body's own signal or another drug cannot switch it on.

Where an agonist turns a receptor on, an antagonist occupies the binding pocket and holds it inactive. A competitive antagonist can be displaced by enough agonist, so its effect depends on the balance of the two; a non-competitive antagonist binds elsewhere or irreversibly and cannot be overcome that way. Naloxone at opioid receptors and most antihistamines at the H1 receptor are everyday antagonists. Neutral antagonists differ from inverse agonists, which push a receptor below its resting activity rather than simply blocking it.

Cannabinoid research has used synthetic CB1 antagonists to map what the endocannabinoid system does, and one, rimonabant, was briefly marketed as a weight-loss drug before withdrawal over psychiatric side effects. CBD has been reported to act as an antagonist at some nonclassical targets such as GPR55 in cell studies, an observation whose clinical meaning has not been established.