Some very smart scientists discovered that cyclic guanosine monophosphate (cGMP) was the key to sustaining an erection. They discovered that when a man gets sexually stimulated, a chain reaction occurs in the tissue of the penis that results in elevated levels of a substance called cyclic guanosine monophosphate (cGMP). As long as there are sufficient levels of cGMP, the penis can remain erect. The more cGMP, the more robust and durable the erection. If you inhibit the degradation of cGMP it stays around in the penis longer, producing a more durable erection. Another way you can think of Levitra (and all other similar medicines) is that it preserves the elevated levels of cGMP that are created when a man is sexually stimulated for a stronger, more lasting erection. That's how Levitra works - Levitra blocks the enzyme phosphodiesterase-5 (PDE5) which is responsible for the neutralization of cGMP. Levitra (Vardenafil) is a highly selective inhibitor of PDE5. (PDE5 is not the only phosphodiesterase involved in the erectile physiology but it appears to be the most important one). The nice thing about PDE5 blockers is that they shouldn't cause erections at inappropriate times because they only block degradation of cGMP produced in reaction to sexual stimulation, such as thinking about something sexy. If you are not being sexually stimulated, there is no cGMP to protect, so the drug remains in the background.

Courtesy drugs.com