L-arginine is an amino acid naturally present in your body. It helps make nitric oxide. Nitric oxide relaxes blood vessels to facilitate a successful erection and is essential for healthy sexual functioning. Researchers studied the effects of L-arginine on ED. Thirty-one percent of men with ED taking 5 grams of L-arginine per day experienced significant improvements in sexual function. A second study showed that L-arginine combined with pycnogenol, a plant product from tree bark, restored sexual ability to 80 percent of participants after two months. Ninety-two percent had restored sexual ability after three months. Another placebo-controlled study found that L-arginine in combination with other medications was well-tolerated, safe, and effective for mild-to-moderate ED.