A phosphodiesterase type 5 inhibitor
) is a drug used to block the degradative action of cGMP-specific phosphodiesterase type 5 (PDE5) on cyclic GMP in the smooth muscle cells lining the blood vessels supplying the corpus cavernosum of the penis. These drugs are used in the treatment of erectile dysfunction
and were the first effective oral treatment available for the condition. Because PDE5 is also present in the arterial wall smooth muscle within the lungs, PDE5 inhibitors have also been explored for the treatment of pulmonary hypertension
, a disease in which blood vessels in the lungs become overloaded with fluid, usually as a result of failure of the left ventricle of the heart.