*36-hour CIALIS is the only prescription erectile dysfunction (ED) tablet clinically proven to both go to work fast, in as little as 30 minutes for some men, and work up to 36 hours.† With CIALIS, you can be ready when the moment is right. Cialis relaxes muscles and increases blood flow to particular areas of the body. It is a phosphodiesterase inhibitor. Cialis is used to treat erectile dysfunction (impotence). It works by helping to increase blood flow into the penis during sexual stimulation. This helps you to achieve and maintain an erection. Cialis may also be used for purposes other than those listed in this medication guide.