There are many drugs that contain Sildenafil Citrate, which are similar in terms of effectiveness, mechanism and intake specifications.Zenegra should only be used when it is needed and should not to be taken on a regular basis.Sildenafil, which is used to stimulate the erectile tissues of the penis, causes an increase in blood flow and erectile function. tablets contain sildenafil. The enzyme guanylate-cyclase is activated when sexual stimulation occurs. This enzyme increases the levels of a chemical called "cyclic guanosine monphosphate" (cGMP), which relaxes blood vessels in penis, allowing blood to flow into the spongy tissues and causing an erection. A second enzyme, phosphodiesterase 5 (PDE5), blocks cGMP from entering the penis. This prevents blood flow and causes an inability to erection. Zenegratablets block the action of PDE5, which causes erectile dysfunction. It allows the natural process for sexual stimulation to activate cGMP, which is responsible for maintaining and achieving erection.