Phentolamine is a competitive nonselective alpha adrenergic receptor antagonist. When given by injection causes blood vessels to expand, thereby increasing blood flow. When injected into the penis (intracavernosal), it increases blood flow to the penis, which results in an erection.

It's primary stocking purpose in hospitals is in crash carts to counteract severe peripheral vasoconstriction secondary to extravasation of peripherally placed vasopressor infusions, such as nor-epinephrine primarily and alpha 1 (one) agonist, vasopressin, etc. Epinephrine (or Adrenaline) infusions are less vasoconstrictive then nor-epinephrine (or noradrenaline) due to the primary beta receptor stimulation before alpha but remains dose dependent.