Bosentas 125 Tablet is used in the treatment of pulmonary arterial hypertension (PAH). Bosentas 125 Tablet is an endothelin receptor blocker. It blocks the action of endothelin (a natural substance) that causes blood vessels to narrow (constrict). This narrowing increases the pressure in the blood vessels of the lungs. Bosentas 125 Tablet helps relax these blood vessels and increases the supply of blood to the lungs.