Phosphodiesterase (PDE)-5 inhibitors, such as sildenafil (Viagra), which was developed in 1998, revolutionized the treatment of the major sexual dysfunction affecting men—erectile disorder. Two congeners have since come on the market—vardenafil (Levitra) and tadalafil (Cialis). All have a similar method of action and have changed people’s expectations of sexual functioning. Although indicated only for the treatment of male erectile dysfunction, there is anecdotal evidence of their being effective in women. They are also being misused as recreational drugs that are believed to enhance sexual performance. These drugs have been used by more than 20 million men, if not more, around the world.

The development of sildenafil provided important information about the physiology of erection. Sexual stimulation causes the release of the neurotransmitter nitric oxide (NO), which increases the synthesis of cyclic guanosine monophosphate (cGMP), causing smooth muscle relaxation in the corpus cavernosum that allows blood to flow into the penis and that results in turgidity and tumescence. The concentration of cGMP is regulated by the enzyme PDE-5, which, when inhibited, allows cGMP to increase and enhance erectile function. Because sexual stimulation is required to cause the release of NO, PDE-5 inhibitors have no effect in the absence of such stimulation, an important point to understand when providing information to patients about their use. The congeners vardenafil and tadalafil work in the same way, by inhibiting PDE-5, thus allowing an increase in cGMP and enhancing the vasodilatory effects of NO. For this reason, these drugs are sometimes referred to as NO enhancers.