Adrenergic antagonist is a pharmaceutical substance that acts to inhibit the action of catecholamines at the adrenergic receptors. It is thus a type of sympatholytic. Many important and commonly prescribed medications are adrenergic antagonists, including prazosin and propanolol. Like all pharmacological receptor antagonists, adrenergic antagonists only show an effect when the receptor's effector is present. It has the opposite effect as adrenergic agonists. They can be divided into alpha blockers and beta blockers.