Prazosin in hypertension. Part II. Effects of the initial dose

The events after the first dose of prazosin have been studied. Twenty-four patients with hypertension were given a single 1 mg tablet. Supine and erect blood pressures were recorded at 15-minute intervals for up to 270 minutes. Six had no previous therapy, 18 were uncontrolled on other drugs, chiefly thiazides or beta-adrenergic blockers or a combination of both. In 10 patients in whom no symptoms of postural hypotension occurred, there was a mean maximum erect diastolic blood pressure fall of 21/14mmHg at an average of 110 minutes after the dose. The remaining 14 had symptoms of postural hypotension of which four were milk, seven moderate and three severe. The mean maximum erect diastolic blood pressure reduction in this group was 67/52mmHg at a mean of 90 minutes after the dose. Subsequent therapeutic response was better in those with an abrupt initial blood pressure fall. Twenty-two further patients were studied in the same manner with a single dose 0.5mg (tablet), 11 of them in a double-blind within-patient crossover study with placebo. Postural symptoms or significant tachycardia did not occur with this dose. Most patients experience some postural hypotension after the first dose of prazosin. Symptoms occur only when this fall is marked, and the extent of the fall is dose related.