To Determine How Different Doses of Tadalafil Work on Getting and Keeping an Erection 26 or 36 Hours After Taking

10 mg tablet taken orally, no more than once a day, at anytime during the 2-4 week equilibration phase and then 24 hours prior to sexual intercourse for 4-6 weeks in the assessment phase

Placebo tablet taken by mouth, no more than once a day, anytime during the 2-4 week equilibration phase and then 24 hours prior to sexual intercourse for 4-6 weeks in the assessment phase

20 mg tablet taken orally, no more than once a day, at anytime during the 2-4 week equilibration phase and then 24 hours prior to sexual intercourse for 4-6 weeks in the assessment phase