Phentolamine is a reversible nonselective alpha-adrenergic antagonist
Adrenergic antagonist
An Adrenergic antagonist is a pharmaceutical substance that acts to inhibit the action of the adrenergic receptors. It is thus a type of sympatholytic.It has the opposite effect as adrenergic agonists.More specifically, they can be divided into:...

Its primary action is vasodilation
Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When...

 due to α1 blockade.

It also can lead to reflex tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...

 because of hypotension and α2 inhibition, which increases sympathetic tone.
The primary application for phentolamine is for the control of hypertensive emergencies
Hypertensive emergency
Malignant hypertension or hypertensive emergency is severe hypertension with acute impairment of an organ system and the possibility of irreversible organ-damage...

, most notably due to pheochromocytoma
A pheochromocytoma or phaeochromocytoma is a neuroendocrine tumor of the medulla of the adrenal glands , or extra-adrenal chromaffin tissue that failed to involute after birth and secretes excessive amounts of catecholamines, usually noradrenaline , and adrenaline to a lesser extent...


It also has usefulness in the treatment of cocaine
Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...

 induced hypertension, where one would generally avoid beta blockers and where calcium channel blockers are not effective.