Cialis is a prescription medicine taken by mouth for the treatment of erectile dysfunction (ED) in men. ED is a condition where the penis does not harden and expand when a man is sexually excited, or when he cannot keep an erection. Cialis may help a man with ED get and keep an erection when he is sexually excited. Cialis (Tadalafil), dubbed “Le Weekend Drug” in Europe, is a new treatment for Erectile Dysfunction (ED) from Lilly ICOS LLC. Clinical studies show that Cialis works faster, lasts up to 36 hours and has less side effects than Viagra.