Sildenafil citrate (Viagra) is a relatively selective 5-phosphodiesterase (PDE) inhibitor. It is the first oral medication approved for the treatment of erectile dysfunction (ED). The neuronal release of NO which binds to the heme-containing region of guanylate cyclase increases levels of cGMP. This leads to a cascade of reaction which results in corporal smooth muscle relaxation and penile erection. Sildenafil causes an erection by inhibiting PDE5, which in turn causes an increase in the intracellular levels of cGMP. Sildenafil is well absorbed after a single oral administration with a t(1/2) of approximately 4 h. The mode of onset varies from 0.5-4 h. The drug has been used in millions of men since first approved by the U.S. FDA 1 year ago and has revolutionized the approach to, and therapy of, erectile dysfunction.