Background: Sexual dysfunction is an important health problem which prevalence in patients with type 2 diabetes (T2D) is higher than other people. The aim of this study was to evaluate the effect of L-arginine oral supplementation on sexual function in men with T2D and mild-to-moderate erectile dysfunction (ED). Methods: Eighty men with T2D, suffering from mild-to-moderate erectile dysfunction participated in this double-blind clinical trial. They were randomly assigned in to two groups; receiving 5 g/day L-arginine (AG) or placebo group (PG) for 4 weeks. Sexual functions were recorded with international index of erectile function questionnaire at the beginning and end
of the intervention. Total testosterone concentrations were measured too. Results: The total mean score of sexual function parameters significantly increased only in AG than PG (43.2 ± 6.2 vs. 39.4 ± 4.0, P = 0.003). Against PG, the mean of all parameters significantly improved in AG after intervention. Only the mean score of erectile function in AG had a significant increase compared to PG. Mean of other sexual activity scores between the two groups had no significant differences. The mean of testosterone concentration had a significant increase in both groups after the intervention, but the mean of changes was significantly higher in AG than in PG (1.91 μg/L vs. 0.53 μg/L, P = 0.004). Conclusions: The present trial showed that daily consumption of 5 g L-arginine for four weeks in T2D men with mild-to-moderate sexual dysfunction improved various parameters such as erectile function, sexual satisfaction, sexual desire, pleasure from sexual intercourse, and testosterone level.