Sildenafil citrate is a selective inhibitor which acts by inhibiting cGMP-specific phosphodiesterase type 5, an enzyme that regulates blood flow in the penis. The sildenafil citrate action mechanism involves the protection of cyclic guanosine monophosphate from degradation by cGMP-specific phosphodiesterase type 5 (PDE5) in the corpus cavernosum. Nitric oxide (NO) in the corpus cavernosum of the penis binds to the receptors of an enzyme called guanylate cyclase, which results in increased levels of cGMP, leading to smooth muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries. This smooth muscle relaxation leads to vasodilation and increased inflow of blood into the spongy tissue of the penis, causing an erection.[

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