- Documented ED for more than 6 months according to the National Institutes of Health (NIH) Consensus statement (the inability to achieve or maintain penile erection sufficient for satisfactory sexual performance). The diagnosis for ED will be confirmed using the five-item version (Rosen RC) of the International Index of Erectile Function (IIEF-5), see Appendix 14.4. The total IIEF-5 score must be at least 8, and not exceed 19. Subjects must have used at least one Phosphodiesterase-5 (PDE-5) inhibitor, such as Viagra, Levitra or Cialis at some time prior to entry in this study.