The main purpose of this study is to determine if tadalafil can slow the decline in walking ability of boys who have Duchenne muscular dystrophy (DMD). The study will also assess the safety of tadalafil and any side effects that might be associated with it in boys who have DMD. Participants will receive study treatment (tadalafil or placebo) for the first 48 weeks of the study, and can then continue into a 48 week extension period during which all participants will receive tadalafil.
