Cialis is an effective, reversible, selective inhibitor of phosphodiesterase type 5 (PDE 5) specific to cyclic guanosine monophosphate (cGMP). When sexual stimulation causes local release of nitric oxide, inhibition of PDE 5 by Cialis leads to increase of cGMP levels in the cavernous body of penis. It results in relaxation of arterial smooth muscles and blood flow to the tissues of the penis that causes erections. Cialis has no effect when sexual stimulation is absent. Cialis improves erection and lets achieve a successful sexual intercourse. The drug acts during 36 hours. If sexual arousal is present the effect appears even after 16 minutes after the medication was taken.