question

Viagra is the brand name for sildenafil citrate, a drug that was originally studied for use in patients with high blood pressure or heart disease. It works by blocking the action of an enzyme called PDE5. The role of this enzyme is to break down another enzyme called cGMP, which relaxes the smooth muscle of the corpus cavernosa (a mass of erectile tissue in the penis), which allows them to become filled with blood and therefore create an erection. When the sildenafil citrate blocks the PDE5, the cGMP levels continue to build so long as the man remains sexually aroused and thus the corpus cavernosa are inflated with blood to a much higher pressure than they would otherwise be. Smooth muscle relaxants have been used to create spontaneous erection as early as the 1980s. But what makes Viagra special is that it acts only when the man is aroused, and only on the penis. This is because the PDE5 enzyme is specific to the penis.