Opsumit (generic name: macitentan) is an endothelin receptor antagonist (ERA) indicated for the long-term treatment of pulmonary arterial hypertension (PAH, WHO Group I) to delay disease progression and reduce hospitalization risk. PAH is a chronic and life-threatening condition characterized by narrowing of the pulmonary arteries, which increases blood pressure in the lungs and strains the heart.

Macitentan works by blocking the action of endothelin-1, a natural substance that causes blood vessel constriction and cell overgrowth in the pulmonary arteries. By preventing endothelin-1 from binding to its receptors, Opsumit helps dilate blood vessels, lower pulmonary artery pressure, improve exercise tolerance, and reduce strain on the right side of the heart.

It is taken orally, usually as a 10 mg tablet once daily, with or without food.
Opsumit is often prescribed alone or in combination with other PAH therapies (such as phosphodiesterase-5 inhibitors or prostacyclin analogs).

Common side effects include headache, anemia, nasopharyngitis, bronchitis, and urinary tract infections. Because of the risk of liver problems and birth defects, regular blood monitoring is required, and women must use reliable contraception during treatment.