What are Viagra, Cialis, and Levitra?
Sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra) belong to a class of drugs known as PDE5 inhibitors. To understand how they work, we’ll need a quick 101 on erections. For an erection to happen, a chemical called nitric oxide (NO) sends a message to the tissues of the penis (in technical terms, the corpus cavernosum), which relax and fill with blood. The erection is ended by another messenger, PDE5 (phosphodiesterase-5).