Grapefruit may interact with tadalafil and lead to unwanted side effects. For this purpose, one dose works for up to 72 hours. Take both tablets at the same time each day. This is not a complete list of side effects and others may occur. Since Cialis is used as needed, you are not likely to miss a dose. It may increase your risk of dizziness or fainting. Tadalafil has been extensively studied for ED.Dosage for erectile dysfunction (ED) For treating erectile dysfunction (ED), Cialis may be used daily or on occasion. Cialis ED dosage for daily use For treating 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 . Generic Cialis (tadalafil): starting at $40 per month (5mg, 10mg, 20mg) Daily generic Cialis (tadalafil): starting at $40 per month (mg, 5mg) Product details.