Glossary of Pharmacology
An antagonist that binds to the receptor in an irreversible manner. The interaction between the antagonist and the receptor is tight (mostly covalent) and therefore the antagonism is insurmountable at any concentration of the agonist.
Phenoxybenzamine is an antagonist of the α-adrenergic receptors in the blood vessels, and it forms a covalent bond with the receptors. Thus, it irreversibly blocks the vasoconstrictive action of catecholamines leading dilation of the blood vessels and reduction in blood pressure.