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An adrenergic blocking agent which produces miosis and a slight reduction in intraocular pressure. It is used mainly to reverse the mydriatic effect of sympathomimetic drugs (e.g. phenylephrine hydrochloride), or even some antimuscarinic drugs (e.g. tropicamide). Common agents include dapiprazole and moxisylyte (thymoxamine). Syn. alpha-blocker. See sympatholytic drugs.