Tazzle is used for erectile dysfunction in men. When sexual stimulation causes the local release of nitric oxide in the corpus cavernosum, nitric oxide activates the enzyme guanylyl cyclase, which increases levels of cGMP. The increased cGMP levels in the corpus cavernosum produce smooth muscle relaxation and inflow of blood into the penile tissues, thereby producing an erection. PDE5 degrades cGMP in the corpus cavernosum, and the inhibition of PDE5 by Tazzle maintains increased cGMP levels in the corpus cavernosum. Tazzle does not affect penile blood flow in the absence of sexual stimulation. Tazzle is used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH). Part of the physiological process of erection involves the release of nitric oxide (NO) in the corpus cavernosum. This then activates the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation in the corpus cavernosum, resulting in increased inflow of blood and an erection. Tazzle is a potent and selective inhibitor of cGMP-specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. This means that, with tadalafil on board, normal sexual stimulation leads to increased levels of cGMP in the corpus cavernosum which leads to better erections. Without sexual stimulation and no activation of the NO/cGMP system, tadalafil should not cause an erection.