Efficacy of Intracavernosal as add-on Therapy to Sildenafil 100 mg on Demand Compared to Sildenafil 100 mg on Demand for the Treatment of Erectile Dysfunction (ED) Not Sufficiently Responsive to Standard Therapy With Phosphodiesterase Type 5 Inhibitors

Inclusion Criteria: Subjects have to fulfill all of the following criteria before being included in the open-label run-in phase: * Written informed consent obtained from the subject * History of ED for at least 6 months prior to screening, defined as "the inability to achieve and maintain an erection of the penis sufficient to complete satisfactory sexual intercourse" (NIH), the diagnosis of ED has to be confirmed by a physician * Understanding of study procedures and willingness to abide by all procedures during the course of the study * Male subject aged ≥18 to ≤ 80 years at visit 1 * Have a monogamous relationship with a female sexual partner (vaginal penetration required for several of the primary efficacy variables) for at least 6 months prior to screening * Highly motivated to obtain treatment for ED * History of previous use of at least 1 marketed PDE5 inhibitor and insufficient therapeutic efficacy despite use of the highest approved dose Subjects have to fulfill all of the following criteria before being included in the double blind treatment phase: * At least 4 attempts at sexual intercourse during the open-label run-in phase with use of 100 mg sildenafil approximately 1 hour before attempting intercourse * IIEF-EF score \<17 * At least 50% of attempts at sexual intercourse during the open-label run-in phase had been unsuccessful i.e. the following question in the Subject Diary had to be answered with "No": "Did your erection last long enough for you to have su…