Tadalafil is a carboline-based compound with vasodilatory activity. Tadalafil selectively inhibits the cyclic guanosine monophosphate (cGMP)-specific type 5 phosphodiesterase- (PDE-5)-mediated degradation of cGMP, which is found in the smooth muscle of the corpus cavernosa and corpus spongiosum of the penis. Tadalafil is used to treat of erectile dysfunction (ED) and either alone or in combination with finasteride for the treatment of benign prostatic hypertrophy (BPH). It is also indicated for the treatment of pulmonary arterial hypertension (PAH) both alone and in combination with macitentan or other endothelin-1 antagonists.