Citrulline is thought to be a proerectile agent because it is a precursor for arginine, and arginine is the substrate from which nitric oxide is produced and then can induce cGMP (via the NO/cGMP/VEGF pathway);[1] an increase in cGMP is also the ultimate effect of PDE5 inhibitors such as sildenafil (also known as Viagra) or icariin from horny goat weed.[2]
In men with erectile dysfunction (as assessed by weak erections and erectile hardness score[3]) who were given 1,500 mg of citrulline daily (two doses of 750 mg) for one month, half of the 24 participants reported a benefit (as assessed by “very satisfied” with treatment), whereas there was an 8.3% improvement in the placebo group.[4]
Citrulline appears to have proerectile properties that occur vicariously through arginine. It is likely more potent than arginine due to greater bodily exposure to arginine, but the preliminary evidence does not suggest that it is more potent than the reference drug sildenafil.