Sildenafil citrate, after consumption of any variants of Generic or branded Viagra enters the body where it inhibits the enzyme called as cGMP specific phosphodiesterase 5 (PDE 5). This enzyme is the key reason behind the restricted flow of the blood to the penis and hence erectile dysfunction. On restraining the action of this enzyme, sildenafil citrate enhances the levels of biochemical nitric oxide and an enzyme called guanylyl cyclase. Increased levels of guanylyl cyclase in turn upsurges the production of a potent vasodilator in the body called cyclic guanosine monophosphate (cGMP). This cGMP enlarges the blood vessels of the penis and then causes enhancement in the influx of the blood to the penis thus resulting in a penile erection which is firm and long lasting thus enabling men to have a satisfactory sexual performance.