Tadalafil belongs to a group of medicines, known as phosphodiesterase-5 (PDE-5) inhibitors, which are used to treat erectile dysfunction or pulmonary arterial hypertension. 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. Tadalafil increases blood flow to particular areas of the body and is used to treat erectile dysfunction. Learn about side effects, interactions and indications. Tadalafil is prescribed for benign prostatic hyperplasia, erectile dysfunction, and pulmonary arterial hypertension. Learn about side effects and more. It works to treat erectile dysfunction by increasing blood flow to the penis during sexual stimulation. This increased blood flow can cause an erection. Tadalafil treats PAH by relaxing the blood vessels in the lungs to allow blood to flow more easily. For people who have ED and BPH (ED/BPH), the recommended starting dose of tadalafil is 5 mg. You’ll usually take this once daily. The recommended starting dose of CIALIS for once daily use is 2.5 mg, taken at approximately the same time every day, without regard to timing of sexual activity. The CIALIS dose for once daily use may be increased to 5 mg, based on individual efficacy and tolerability.