Although not all of these side effects may occur, if they do occur they may need medical attention. This medicine comes with a patient information leaflet. By doing so—combined with sexual stimulation—a cascade of events occurs that increases nitric oxide. However, it stays in your body for up to 36 hours and can be taken every 24 hours, so it can work for you constantly. Summary Cialis and Viagra are both pills for treating erectile dysfunction.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). CIALIS for Use as Needed for Erectile Dysfunction. The recommended starting dose of CIALIS for use as needed in most patients is 10 mg, taken prior to anticipated sexual activity. The dose may be increased to 20 mg or decreased to 5 mg, based on individual efficacy and tolerability. Whichever condition Cialis is used to treat, the recommended maximum dose per day is 20 mg. This is the highest dosage that a doctor may prescribe. Dosage for erectile dysfunction (ED) For.