The sexually competent male must have desire for his sexual partner (libido), divert blood from the iliac artery into the corpora cavernosa to achieve penile tumescence and rigidity (erection) adequate for penetration, then discharge sperm, and prostatic and seminal vesicle fluid through the urethra (ejaculation), and experience a sense of pleasure (orgasm). Impotence is defined as the inability to develop or sustain erection 75 percent of the time. It is a common abnormality and may be due to psychological causes, medications, hormonal abnormalities, neurologic, or vascular problems. 

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MECHANISM OF ACTION - Does not directly cause penile erections, but affects the response to sexual stimulation. The physiologic mechanism of erection of the penis involves release of nitric oxide (NO) in the corpus cavernosum during sexual stimulation. NO then activates the enzyme guanylate cyclase, which results in increased levels of cyclic guanosine monophosphate (cGMP), producing smooth muscle relaxation and inflow of blood to the corpus cavernosum. Cialis enhances the effect of NO by inhibiting phosphodiesterase type 5 (PDE-5), which is responsible for degradation of cGMP in the corpus cavernosum; when sexual stimulation causes local release of NO, inhibition of PDE-5 by tadalafil causes increased levels of cGMP in the corpus cavernosum, resulting in smooth muscle relaxation and inflow of blood to the corpus cavernosum. At recommended doses, it has no effect in the absence of sexual stimulation.

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