Viagra (sildenafil) and Levitra (vardenafil) belong to a class of drugs called PDE5 inhibitors, which are commonly used to treat erectile dysfunction (ED). PDE5 inhibitors work by relaxing blood vessels in the penis and signaling the blood vessels to stay dilated longer. This allows for increased blood flow to the penis and stronger, longer-lasting erections. Other PDE5 inhibitors include tadalafil (brand name Cialis) and avanafil (brand name Stendra). It’s important to note that ED medications are not magic pills–all of these drugs require arousal and sexual stimulation to work effectively.