The aim was to compare the differences between daily 5 mg and on-demand 20 mg tadalafil use in diabetic patients with erectile dysfunction (ED), and the effects of two different tadalafil protocols on ejaculatory and lower urinary tract symptoms (LUTS). Of the 63 diabetic patients with ED, 31 were given 5 mg tadalafil once a day, and 32 were given 20 mg tadalafil on-demand four times a month over three months. Erectile function, erectile hardness, ejaculatory function, and LUTS were assessed at pretreatment, first-and third-month controls. Both tadalafil protocols increased International Index of Erectile Function (IIEF) scores in all patients under 65 years, whereas patients older than 65 years did not benefit. Ejaculatory function, the quality of the hardness of an erection, and LUTS improved in both groups in the study. Tadalafil improved sexual function with acceptable side effects in diabetic men with ED. Both protocols equally improved LUTS and the quality of the erection. Daily use of 5 mg of tadalafil significantly improved the quality of ejaculation and LUTS more than the on-demand use of 20 mg of tadalafil. It may be beneficial to give 5 mg tadalafil daily to patients over 65 years old who do not benefit from treatment with 20 mg of tadalafil or in patients who have LUTS over 65 years old.