Penegra Citrate belongs to a group of medicines called phosphodiesterase type-5 (PDE-5) inhibitors. It works by helping to relax the blood vessels of penis, allowing blood to flow into penis. The physiologic mechanism of erection of the penis involves release of nitric oxide (NO) in the corpus cavernosum during sexual stimulation. NO then activates the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP), producing smooth muscle relaxation in the corpus cavernosum