What Is Cialis? Tadalafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow more easily. Do not take a double dose to make up for a missed one. From Mayo Clinic to your inbox Sign up for free and stay up to date on research advancements, health tips, current health topics, and expertise on managing health. This medicine is available only with your doctor’s prescription. Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand.1. How it works Cialis is a brand (trade) name for tadalafil which may be used to treat erectile dysfunction (ED) and benign prostatic hyperplasia (BPH). Cialis . Tadalafil (Cialis) is used to treat erectile dysfunction (ED, impotence; inability to get or keep an erection), and the symptoms of benign prostatic hyperplasia (BPH; an enlarged . Tadalafil is used to treat men who have erectile dysfunction (also called sexual impotence). Tadalafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. The penis is one of the areas where this enzyme works.