If you become pregnant while taking tadalafil, call your doctor. Do not take a double dose or more than one dose per day to make up for a missed one. What special precautions should I follow? The results showed the combination to be even more effective than either medication alone. It belongs to the drug class known as PDE5 inhibitors. Use only the brand of this medicine that your doctor prescribed. Do not keep outdated medicine or medicine no longer needed.Cialis comes as an oral tablet that’s typically taken once per day or on occasion as needed. The dosage can vary depending on what condition the drug is used to treat. Specifically, Cialis. Cialis is a prescription medication containing the active ingredient tadalafil. Cialis belongs to a group of medications known as phosphodiesterase-5 (PDE-5) inhibitors. Healthcare providers commonly prescribe tadalafil to treat erectile dysfunction (ED). 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.