Antihypertensive effects and plasma levels of guanfacine in man.

The antihypertensive effect and the plasma levels of guanfacine were studied for two months in 25 patients with sustained essential hypertension. In three patients, blood pressure was unchanged. In ten patients, blood pressure returned to the normal range within 15 days, at a daily dose of 2 mg. In 12 patients, increasing doses were necessary to normalize the blood pressure within two months. In these cases, the decrease in systolic pressure was negatively correlated with the daily dose of guanfacine (r = -0.40) and the steady-state plasma level (r = -0.62; P less than 0.01); analysis of the curves showed that the normalization of blood pressure coincided with a plasma drug level of 8 ng/ml, i.e., a 4-mg daily dose. This study demonstrated the efficacy of guanfacine monotherapy in hypertension and suggests that the normalization rate of blood pressure levels does not further increase with daily doses higher than 4 mg.