Macitentan is primarily used in the treatment of pulmonary arterial hypertension (PAH), a condition characterized by high blood pressure in the arteries of the lungs. It works by blocking the action of endothelin, a substance that narrows blood vessels and increases blood pressure. By inhibiting endothelin receptors, macitentan helps to relax and widen the blood vessels in the lungs, improving blood flow and reducing the strain on the heart. This can lead to improved exercise capacity and a slower progression of the disease. It is often prescribed for long-term management and is typically taken orally once daily. Patients using macitentan require regular monitoring to assess its effectiveness and to manage potential side effects, such as liver function abnormalities or anemia.