Do not double doses. Make sure you consult with a doctor before taking tadalafil, and do not change the dosage that your doctor recommends. 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. Do not eat grapefruit or drink grapefruit juice while you are using this medicine. If your dose is different, do not change it unless your doctor tells you to do so. Also, do not stop taking this medicine without checking first with your doctor.Cialis (tadalafil) increase increases blood flow to a certain area of the body and is used to treat erectile dysfunction (impotence). Includes Cialis side effects, interactions and indications.  · 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. Coadministration with finasteride for BPH: Tadalafil 5 mg plus finasteride 5 mg PO once daily for ≤26 weeks; incremental benefit of tadalafil decreases from 4 weeks until 26 weeks, and the benefit beyond 26 weeks is unknown BPH or BPH/ED (CrCl mL/min): mg/day starting dose recommended; may increase to 5 mg according to.