Phentolamine is an alpha-adrenoceptor antagonist, which means that it blocks chemicals that bind to alpha-adrenergic receptors. Competitively binding to their receptors, phentolamine causes muscle relaxation and a widening of blood vessels. When blood vessels are widened, blood pressure decreases, and blood flow into the penis increases. Phentolamine also blocks 5-HT receptors, which cause the release of histamine from mast cells.