Tadalafil works by blocking the activity of an enzyme called phosphodiesterase 5 (PDE5). This enzyme usually stops some types of muscles from relaxing, including the muscles lining some types of blood vessels. By blocking PDE5, the muscles will relax and blood flow increases. For erectile dysfunction, it increases blood flow to the penis when aroused. This makes it easier to get and keep an erection. For pulmonary arterial hypertension, tadalafil relaxes the blood vessels in the lungs. This lowers blood pressure and reduces strain on the heart. For benign prostatic hyperplasia, tadalafil relaxes the muscles of the bladder and prostate, making it easier to urinate and reducing symptoms.