Selectively inhibits cGMP-specific phosphodiesterase type 5. The mechanism of penile erection involves the release of nitric oxide into the corpus cavernosum of the penis during sexual stimulation. Nitrous oxide activates cGMP, which causes smooth muscle relaxation allowing the flow of blood into the corpus cavernosum. Phosphodiesterase type 5 inhibitors prevent the breakdown of cGMP by phosphodiesterase, leading to increased cGMP levels and prolonged smooth muscle relaxation promoting the flow of blood into the corpus cavernosum. Relaxes blood vessels in the lungs in patients with pulmonary hypertension, improving exercise tolerance.