Comparison of effects of guanfacine and clonidine on blood pressure, heart rate, urinary catecholamines, and cyclic nucleotides during and after administration to patients with mild to moderate hypertension.

In a random trial, the effects of treatment and withdrawal of guanfacine were compared with those of clonidine in 20 uncomplicated hypertensive patients. Elevated blood pressure returned to normal or responded well in all the patients given either guanfacine once daily or clonidine thrice daily. The pulse rate was reduced comparably by both treatments after 12 weeks, but the effect of guanfacine developed more gradually. Both guanfacine and clonidine significantly inhibited urinary noradrenaline, dopamine, and cyclic nucleotide excretion, while urinary adrenaline levels were unaffected. Side effects occurred earlier during treatment with clonidine. After sudden withdrawal, all the parameters tended to increase gradually in the guanfacine group, reaching base line by days 4-6. In the clonidine group the increase was more rapid, with pretreatment values reached within the 2nd day, and sometimes these values were surpassed. After withdrawal of clonidine all the patients had one or more side effects, most of them occurring within 48 h, while only 60% of the patients in the guanfacine group reported the appearance of unwanted symptoms, on days 3-6. It is concluded that there are close similarities between the effects of guanfacine and clonidine on the parameters evaluated, except for dopamine excretion, which was significantly less affected by guanfacine. Marked differences were found after abrupt withdrawal, with guanfacine less likely to produce the "discontinuation syndrome," probably due to its long half-life.