Cialis operates similarly to various other PDE5 inhibitors in the therapy of ED. Viagra, Levitra, and Cialis are all PDE5 inhibitors. As with other ED drugs, Cialis is only efficient during sexual excitement. viagradrugstore.com The simple act of taking Cialis will certainly not cause an erection in males. Maybe Cialis can best be defined by contrasting as well as contrasting it with its competitors, Viagra as well as Levitra: