Objective To investigate the effectiveness and safety of a tadalafil 5-mg single daily dose in the treatment of lifelong premature ejaculation (PE). Patients and Methods A total of 55 men with lifelong PE were enrolled in this study. The intravaginal ejaculatory latency time (IELT) and Premature Ejaculation Diagnostic Tool (PEDT) were used to assess the efficacy of tadalafil. Baseline recording of the IELT (using a stopwatch) and PEDT was done. Patients were randomised to take a single daily dose of tadalafil 5 mg or placebo. Re-assessment was done by IELT and PEDT after 1 and 3 months. Results As regard, age, number of children, frequency of intercourse and duration of PE there are no statically significant differences between both groups. In both the tadalafil 5-mg and placebo groups the IELT statically significantly increased after 1 month by a mean (SD) of 4.8 (4.7) and 5 (4.9) s, and after 3 months by a mean (SD) of 6.6 (7) and 4.5 (9) s; and the PEDT scores significantly improved after 1 month by a mean (SD) of 2.1 (1.9) and 2 (1.5), and after 3 months by a mean (SD) of 3.5 (2.5) and 3.3 (2.2). However, there is was no statically significant difference between the tadalafil 5-mg and placebo groups for the IELT and PEDT at baseline or after 1 and 3 months of treatment. Conclusions Despite 5-mg tadalafil once daily being safe and it showed only a slight improvement in IELT and PEDT in the treatment of lifelong PE, failing to show any statistically significant difference from the placebo drug. Abbreviations : ED: erectile dysfunction; 5-HT: 5-hydroxytryptamine (serotonin); IELT: the intravaginal ejaculation latency time; ISSM: International Society of Sexual Medicine; PE: premature ejaculation; PEDT: Premature Ejaculation Diagnostic Tool; PDE5i: phosphodiesterase-5 inhibitor; SSRI: selective serotonin reuptake inhibitor