Erection requires a complex chain of events starting from nervous system signals and ending by the release of "chemical messengers", one of those is called cyclic GMP (cGMP).
cGMP directs the blood vessels in the penis, so penis becomes erect.
cGMP is normally blocked by another chemical in the body called phosphodiesterase type 5(PDE5 mentioned in Main section).
The erectile function enhances by inhibition of PDE5 that increases the amount of cGMP. So Tadalafil (Cialis®) basically inhibits PDE5. However, tadalafil's inhibition of PDE5 will have no effect without direct sexual stimulation of the penis.
