(Sildenafil citrate) is the citrate salt of Sildenafil, a selective inhibitor of cyclic guanosine monophosphate (cGMP)-sceptic phosphodiesterase type 5(PDE5). The physiologic mechanism of erection of the penis involves release of (NO) nitric oxide in the corpus cavernosum during sexual stimulation. Nitric oxide then activates the enzyme guanylate cyclase, which results in increased levels of (cGMP) cyclic guano sine monophosphate, producing smooth muscle relaxation in the corpus cavernosurn and allowing inflow of blood. Sildenafil upgrade the effect of NO by inhibiting phosphodiesterase type 5 (PDE5), which is responsible for degradation of cyclic guano sine monophosphate in the corpus cavernosum. Sildenafil has no direct relaxant effect on isolated human corpus cavernosum. When sexual stimulation causes local release of nitric oxide, inhibition of PDE5 by sildenafil causes increased levels of cyclic guano sine monophosphate in the corpus cavemosum, resulting in smooth muscle relaxation and inflow of blood to the corpus cavernosum. Sildenafil at recommended doses has no effect in the absence of sexual stimulation.