Sildenafil Citrate is the active element of Viagra. It is the third member of the phosphodiesterase type 5 (PDE-5) inhibitors category of medications. Sildenafil Citrate works to block the action of the PDE-5 enzyme that regulates the penile erections. Increased level of PDE-5 is known to result in restricted flow of blood to penis which in turn results into ED. Thus, the restricted PDE-5 production leads to increase in the levels of the nitric oxide and guanylylcyclase. They cause elevated synthesis of cGMP near the penile region. cGMP is known to cause vasodilation, it is a potent vasodilator in the body. Therefore, increased production of cGMP widens the blood vessels and arteries that carry the blood to the penis from the heart. Under the cGMP influence the smooth muscle in penis relax and allow the retention of more blood in the penile arteries. This results in full-fledged and stiffer penile erection which is sustained for the longer time required for satisfactory sexual intercourse.