Sildenafil is one of selective phosphodiesterase type 5 inhibitors. It is used to block the degradative action of phosphodiesterase type 5 on cyclic GMP in the smooth muscle cells lining the blood vessels, which leads to smooth muscle relaxation in blood vessels and increases blood flow. Because phosphodiesterase type 5 is also present in the arterial wall smooth muscle within the lungs, phosphodiesterase type 5 inhibitors is also used for the treatment of pulmonary hypertension.