Tadalafil stops the breakdown of a chemical responsible for relaxing muscles. What Is Cialis? Cialis is a brand-name prescription medication. If you miss a dose of Adcirca or Alyq, take the medicine as soon as you can, but skip the missed dose if it is almost time for your next dose. Grapefruit may interact with tadalafil and lead to unwanted side effects.Tadalafil (Cialis) is a phosphodiesterase-5 enzyme (PDE-5) inhibitor. It works by blocking PDE This is a protein in your body that breaks down the molecules that normally cause an erection. When it blocks PDE-5, tadalafil (Cialis) helps an erection occur by allowing certain muscles in the penis to relax and the penis to fill with blood. Last updated on Uses Warnings Before taking Dosage Side effects Interactions FAQ What is Cialis? Cialis relaxes muscles of the blood vessels and increases blood flow to particular areas of the body. Cialis is used to treat erectile dysfunction (impotence) and symptoms of benign prostatic hypertrophy (enlarged prostate).  · Taking higher dosages of Cialis than this, such as 40 mg per day, raises your risk for serious side effects. Examples of these side effects include very low blood pressure and priapism (a.