Tadalafil is the active ingredient in Cialis. Tadalafil is a prescribed drug that is used primarily to treat erectile dysfunction (ED), also known as impotence, which is the inability to get or maintain an erection satisfactory for intercourse. Tadalafil can also be used to treat symptoms of benign prostatic hyperplasia (BPH), which is an enlarged prostate, as well as pulmonary arterial hypertension.