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Sildenafil, usually sold under the brand name Viagra, belongs to a group of medicines called PDE‑5 inhibitors. When a man is aroused sexually, his body releases a chemical that causes a molecule called cGMP to build up.
This makes the smooth muscles of the penis relax, allowing more flow of blood to flow to the penis, which causes an erection. Viagra affects this process by blocking an enzyme called PDE‑5, which normally reduces cGMP.
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