Sildenafil citrate is widely used as an effective and safe oral treatment for erectile dysfunction of various etiologies. Moreover, it is a potent and selective inhibitor of phosphodiesterase type 5 enzymes that acts to break down cyclic guanosine monophosphate (cGMP) (the main effector of NO action). So, this medication not only amplifies the effect of sexual stimulation by retarding the degradation of this enzyme but also play an important actions through augment NO effects on various body structures and used in treatment of certain chronic diseases (endothelial dysfunction, pulmonary hypertension, and heart failure).