Drugs five years later: clonidine.

Clonidine represents the prototype of a new class of centrally acting antihypertensive agents, classed as partial alpha-adrenergic antagonists. Blood pressure reduction is characterized, hemodynamically, by reduced cardiac output with unchanged peripheral vascular resistance at rest. Reflex control of blood pressure during orthostasis and exercise appears to be unimpaired, and orthostatic hypotension is uncommon. As with most other antihypertensive agents, satisfactory reduction of blood pressure with clonidine given as a sole agent is limited to patients with relatively mild hypertension; an additive or synergistic effect of diuretic administration has been well documented. Abrupt withdrawal of clinidine has been reported to be followed, within 24 to 36 h, by rebound hypertension, tachycardia, cardiac arrhythmias, and other changes suggestive of sympathetic overactivity. The incidence and clinical significance of rebound hypertension after abrupt cessation of clonidine therapy, and indeed the profile of blood pressure responses to varying physical activity during therapy, remain to be evaluated.