Clonidine and guanfacine in hypertension.

Guanfacine, 1 to 3 mg/day, and clonidine, 0.1 to 0.3 mg twice a day, were compared in a 24-week double-blind, randomized, parallel study of 42 patients with hypertension that was inadequately treated by chlorthalidone, 25 mg/day. Mean reduction of blood pressure was 18/9 mm Hg after guanfacine and 14/8 mm Hg after clonidine. To determine the incidence of rebound hypertension, subjects were hospitalized for 7 days during chlorthalidone therapy for collection of baseline data and once again immediately after abrupt withdrawal of the alpha-agonist after 24 weeks of dosing. Although blood pressure and heart rate rose significantly in both groups, the changes after clonidine withdrawal were greater and occurred earlier (day 2) than those after guanfacine withdrawal (day 4). Forty percent of the subjects receiving guanfacine and 64% of subjects receiving clonidine had diastolic blood pressure elevations greater than or equal to 10 mm Hg from baseline. There were increases in urinary norepinephrine levels in both groups after drug withdrawal, but these correlated poorly with blood pressure rise. Side effects after guanfacine were much the same as those after clonidine. Guanfacine taken once a day provides an effective and safe alternative to clonidine in the management of essential hypertension.