Tadalafil is a selective, reversible inhibitor of cyclic guanosine monophosphate (cGMP) - a specific phosphodiesterase type 5 (PDE 5). When sexual stimulation causes local release of nitric oxide, inhibition of PDE 5 by tadalafil produces increased levels of cGMP in the corpus cavernosum. This leads to relaxation of smooth muscle and blood flow to the tissues of the penis, thereby creating an erection. Tadalafil does not exert its effect in the treatment of erectile dysfunction in the absence of sexual stimulation.