Oral phosphodiesterase 5 (PDE5) inhibitors for the treatment of erectile dysfunction are preferred by most men, and are recommended in guidelines as first-line therapy, because of convenience, high efficacy, and low rates of side effects. Tadalafil (Cialis) is a new agent that has been studied in different patient populations. It has a different molecular structure than other PDE5 inhibitors, and a different pharmacologic profile that provides a longer period of effectiveness than other agents. This article will review clinical trials on tadalafil, to provide a comprehensive overview of its efficacy and safety.