Cialis^® (tadalafil) is a prescription medication used to treat erectile dysfunction (ED), or impotence. The drug works by blocking a chemical that reverses an erection by allowing blood to flow out of the penis. In previous clinical studies, up to 78 percent of men reported that Cialis allowed them to have successful intercourse. In most cases, Cialis starts working within an hour; and it can work up to 36 hours after you take your dose. A few common side effects of Cialis include nasal congestion, indigestion, and pain in the arms or legs. Currently, no generic Cialis is available for sale.