Tadalafil has been extensively studied for ED. You need a prescription from a doctor to buy tadalafil. With PAH, high blood pressure develops when a blockage in the arteries within the lung limits normal blood flow. ED occurs more frequently in older men. Counterfeit medicines can be very dangerous. · The approved dosages of Cialis for ED, BPH symptoms, or both, are between mg and 20 mg a day or as needed. Dosages higher than 20 mg a day haven’t been studied, and it’s not known what. How Cialis Works. Tadalafil and other PDE5 inhibitors primarily work by inducing muscle relaxation. Tadalafil stops the breakdown of a chemical responsible for relaxing muscles. In ED and PAH, the drug targets muscle cells in blood vessels. When the blood vessel muscle cells relax, this expands the vessel and increases blood flow; this . CIALIS for Use as Needed. Creatinine clearance 30 to 50 mL/min: A starting dose of 5 mg not more than once per day is recommended, and the maximum dose is 10 mg not more than once in every 48 hours. Creatinine clearance less than 30 mL/min or on hemodialysis: The maximum dose is 5 mg not more than once in every 72 hours [see .