In 1998, Viagra was the first effective oral medication to show efficacy in the treatment of ED. After certain time Levitra and Cialis were approved by the FDA. All three of these medications work in a similar manner by blocking the breakdown of the enzyme, cyclic GMP, which is responsible for the smooth muscle relaxation and trapping blood in the penile tissue when a man is sexually excited. But there are also some side effects of using the ED drugs like Viagra, levitra and cialis...