Bosentan is used to treat the symptoms of pulmonary arterial hypertension. This is the high blood pressure that occurs in the main artery that carries blood from the right side of the heart (the ventricle) to the lungs. When the smaller blood vessels in the lungs become more resistant to blood flow, the right ventricle must work harder to pump enough blood through the lungs. Bosentan works by blocking a hormone (a naturally occurring substance), that is found in the blood and lungs in large quantities of the people with pulmonary arterial hypertension. Bosentan helps by increasing the supply of blood to the lungs and reducing the workload of the heart.