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Summary: Earlier nonselective [alpha]1-adrenergic blocking drugs such as phentolamine and phenoxybenzamine are now restricted to the pharmacological management of [alpha]1-adrenergic crisis and phaeochromocytoma. Prazosin, the first selective [alpha]1-blocker approved for the treatment of hypertension, became available in the mid-1970s. Additional [alpha]1-blockers such as doxazosin and terazosin have been introduced during recent years. The undesirable effects of all members of this class are similar. Most adverse events can be attributed to reversible competitive antagonism of postsynaptic [alpha]1-adrenergic receptors in tissues that sustain high levels of [alpha]-adrenergic sympathetic tone, e.g. resistance arteries, capacitance veins and the urinary bladder outflow tract. Orthostatic hypotension with a sensation of intense faintness and occasional syncope, can occur shortly after the initial dose. Aggravating factors include upright posture, intravascular volume depletion and concurrent administration of other medications that lower blood pressure, including all other classes of antihypertensive drugs. The problem is reduced or avoided by the choice of low starting doses, beginning treatment at bedtime and by minimising other risks.

Among overall adverse effects, asthenia, dizziness, faintness and syncope predominate and occur in 10 to 20% of patients, leading to discontinuation of therapy in about half that number. Infrequent adverse events include headache, drowsiness, palpitations, urinary incontinence and priapism. Some patients experience a 1 to 2kg bodyweight gain which may be associated with secondary hyperaldosteronism. Tolerance appears to develop to the benefits of [alpha]1-blockade in patients with congestive heart failure, but not in hypertension. Two unexpected effects, improved urinary flow in men with benign prostatic hyperplasia and increased (i.e. improved) high density lipoprotein (HDL) cholesterol in hypertensive patients with dyslipidaemia, provide therapeutic benefits in these settings.

Copyright 1994 Adis International