Vardenafil is an oral drug used to treat impotence, the inability to attain or maintain a penile erection. It has a mechanism of action similar to sildenafil (Viagra) and tadalafil (Cialis). Penile erection is caused by the engorgement of the penis with blood. This engorgement occurs when the blood vessels delivering blood to the penis increase in size and increase the delivery of blood to the penis. At the same time, the blood vessels carrying blood away from the penis decrease in size and decrease the removal of blood from the penis. Sexual stimulation that leads to engorgement and erection causes the production and release of nitric oxide in the penis. Nitric oxide activates the enzyme guanylate cyclase to produce cyclic guanosine monophosphate (cGMP). The cGMP is primarily responsible for increasing and decreasing the size of the blood vessels carrying blood to and from the penis, respectively. Vardenafil prevents an enzyme called phosphodiesterase-5 (PDE-5) from destroying cGMP so that cGMP persists longer. The longer cMP persists, the more prolonged the engorgement of the penis. The FDA approved vardenafil in August 2003.