Pharmacology and pharmacokinetics of minoxidil.

Minoxidil is a potent oral vasodilator acting on the arteriolar side of the circulation. Reflex tachycardia and sodium retention are consequences of increased sympathetic activity. Its biotransformation is 90% hepatic, with no evidence of accumulation of minoxidil when it is given chronically in patients with either normal renal function. The half-life is approximately 3 hr, regardless of whether the dose is given singly or on a multiple-dose basis. It is widely distributed throughout the body, and hence its volume of distribution is greater than 200 liters. There appears to be a dissociation between the peak plasma concentration and the antihypertensive response owing to the fact that minoxidil rapidly leaves the plasma on its way to the principal site of action, that is, the vascular smooth-muscle receptor site. No tolerance or refractoriness appears to develop with long-term minoxidil administration.