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Abstract
Penile erection is mediated by nitric oxide (NO) and its second messenger cyclic guanosine monophosphate (cGMP). The activity of cGMP is modulated by the enzyme phosphodiesterase (PDE). The most important physiological isoform of the enzyme in the penis is PDE5. Sildenafil is a specific inhibitor of this enzyme which potentiates nitrergic cavernosal relaxation in vitro and in animals. Clinical studies suggest that it is effective in the treatment of male erectile dysfunction (MED), which would make it the first orally active therapy for this condition.