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Sold under the brand name Viagra, sildenafil is a reversible, selective phosphodiesterase type 5 (PDE5) inhibitor and a proven ED medication that has been used by millions of men. Sexual stimulation is required to initiate the local release of nitric oxide so the inhibition of PDE5 by sildenafil has no effect in the absence of sexual stimulation. Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and corpus cavernosum smooth muscle. This response is mediated by the release of nitric oxide (NO) from nerve terminals and endothelial cells, which stimulates the synthesis of cyclic guanosine monophosphate (GMP) in smooth muscle cells. Cyclic GMP causes smooth muscle relaxation and increased blood flow into the corpus cavernosum. The inhibition of PDE5 enhances erectile function by increasing the amount of cyclic GMP.