Supplementation with L-citrulline in a dose of 1½ grams daily for 1 month was shown to improve ED in a placebo-controlled, single blind study of 24 men with mild ED. Only 2% of the placebo group showed an improved erectile hardness score compared to 50% of the L-citrulline group. The L-citrulline group also showed an increase in the mean number of intercourses per month.

This study was conducted by Cormio L, De Siati M, Lorusso F, Selvaggio O, Mirabella L, Sanguedolce F, Carrieri G at the University of Foggia, Italy

Their conclusion, "Although less effective than phosphodiesterase type-5 enzyme inhibitors, at least in the short term, L-citrulline supplementation has been proved to be safe and psychologically well accepted by patients. Its role as an alternative treatment for mild to moderate ED, particularly in patients with a psychologically fear of phosphodiesterase type-5 enzyme inhibitors, deserves further research."