Study the Safety and Effectiveness of Tadalafil in Men With Problems Getting or Maintaining an Erection When Taken Prior to Desiring an Erection

5 mg tadalafil tablet, by mouth, as needed, no more than 1 dose per day, for 12 weeks

placebo tablet taken by mouth, as needed, no more than 1 dose per day, for 12 weeks

10 mg tadalafil tablet, by mouth, as needed, no more than 1 dose per day, for 12 weeks

20 mg tadalafil tablet, by mouth, as needed, no more than 1 dose per day, for 12 weeks