Cialis (Tadalafil). Oral medications such as Cialis, Levitra and Viagra are available for the treatment of erectile dysfunction. They are in a class called PDE 5 inhibitors. They work to restore a normal system of blood flow in the penis so that an erection can occur with sexual stimulation.Highlights of Cialis: Only ED medicine clinically proven to work up to 36 hours. Works as fast as 30 minutes in some men.hey work to restore a normal system of blood flow in the penis so that an erection can occur with sexual stimulation. Highlights of Cialis: Only ED medicine clinically proven to work up to 36 hours. Works as fast as 30 minutes in some men.