PDE5 inhibitors are an effective treatment for erectile dysfunction. Viagra (Sildenafil), Levitra (Vardenafil), Cialis (Tadalafil) and Spedra (Avanafil) are all PDE5 inhibitors used to treat erectile dysfunction. Sexual stimulation causes a release of Nitrous Oxide into the blood vessels of the penis. This causes blood vessel walls to relax, and the blood flow to increase, leading to an erection. PDE5 inhibitors reduce the degradation of one of the intermediate chemicals that causes the blood vessel walls to relax. Therefore, PDE5 inhibitors help to maintain sufficient blood flow for an erection.