Figure 1: Study design - Randomized patients received 8 weeks of treatment with tadalafil or sildenafil PRN. After 8 weeks of treatment, followed by a wash-out period of 7 to 10 days, the patients crossed over to the opposite treatment for another 8 weeks followed by an 8-week extension period. Abbreviations: DRAQ: drug attribute questionnaire; IIEF: international index of erectile function; PAIRS: psychological and interpersonal relationship scale; PITPQ: phosphodiesterase 5 inhibitor treatment preference questionnaire; PRN: pro re nata (on demand); QoL: quality of life; SEP: sexual encounter profile; SLQQ: sexual life quality questionnaire; THX: treatment satisfaction; V: visit. aWash-out period is 7 to 10 days; up to 10 days have been included in the timeline (i.e., 1.5 weeks). The number of weeks has been rounded up to the nearest integer. bAt screening and baseline, the patient and partner answered the 10-item QoL domain, as the study treatment was not administered. At the end of the extension phase, the complete SLQQ was administered, both QoL and THX domain.