How to take norethisterone to delay period 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 the 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 let’s achieve a successful sexual intercourse. The drug acts for 36 hours. If sexual arousal is present the effect appears even after 16 minutes after the medication was taken. Many people using this medicine do not have serious side effects.