Cialis contains the active ingredient Tadalafil that belongs to a group of ED preparations called PDE5 inhibitors. Therapeutic effect of the medication is associated with inhibiting enzyme called phosphodiesterase type 5. This enzyme is responsible for the breaking down of the cyclic GMP. By inhibiting this enzyme Cialis increases the amount of cGMP in the blood vessels of the penis. This results in the relaxation of the blood vessels in the cavernous tissue and increasing blood flow to the penis. Cialis improves erection and the ability to perform a successful sexual intercourse.