Tadalafil is approved for the treatment of men who experience difficulty having and maintaining an erection (impotence). Tadalafil is a generic prescription drug that is used to treat several conditions, namely erectile dysfunction (impotence), as well as enlarged prostate (benign prostatic hyperplasia or BPH) and high blood pressure in the lungs (pulmonary arterial hypertension). Tadalafil (like its brand-name counterparts, Cialis® and Adcirca®) belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors. These medicines prevent the enzyme PDE5 from working too quickly. The penis is one of the body parts where this enzyme works.
Source: FDA.gov