Clinical Drug Experience Knowledgebase

Phenoxybenzamin (marketed under the trade name Dibenzyline) is an alpha-adrenergic antagonist with long duration of action. It is indicated in the treatment of pheochromocytoma, to control episodes of hypertension and sweating. If tachycardia is excessive, it may be necessary to use a beta-blocking agent concomitantly. Phenoxybenzamine produces its therapeutic actions by blocking alpha receptors, leading to a muscle relaxation and a widening of the blood vessels. This widening of the blood vessels results in a lowering of blood pressure. Phenoxybenzamine hydrochloride can produce and maintain “chemical sympathectomy” by oral administration. It increases blood flow to the skin, mucosa and abdominal viscera, and lowers both supine and erect blood pressures. It has no effect on the parasympathetic system. Twenty to percent of orally administered phenoxybenzamine appears to be absorbed in the active form. The half-life of orally administered phenoxybenzamine hydrochloride is not known; however, the half-life of intravenously administered drug is approximately 24 hours. Demonstrable effects with intravenous administration persist for at least 3 to 4 days, and the effects of daily administration are cumulative for nearly a week. The following adverse reactions have been observed, but there are insufficient data to support an estimate of their frequency: Postural hypotension, tachycardia, inhibition of ejaculation, nasal congestion, and miosis. These so-called “side effects” are actually evidence of adrenergic blockade and vary according to the degree of blockade. Miscellaneous: Gastrointestinal irritation, drowsiness, fatigue.   ^NCATS