How nitric oxide supports blood flow, how L-arginine and L-citrulline differ, what the erection studies found, and the safety cautions for NO supplements.
Nitric oxide (NO) relaxes blood vessels and is made in the body from the amino acid L-arginine. L-arginine has a 2019 meta-analysis suggesting benefit for mild to moderate erectile dysfunction at 1,500 to 5,000 mg a day, and L-citrulline (which the body converts to arginine) has one small pilot study. Both can lower blood pressure and interact with nitrates and ED drugs, so check with your doctor first. These supplements are not a treatment for erectile dysfunction; see a doctor about persistent symptoms.
Nitric oxide is a signaling molecule made by the lining of blood vessels from L-arginine. It tells the surrounding muscle to relax, widening the vessel and increasing blood flow. This is the same pathway that prescription ED drugs act on, which is why NO-supporting supplements are marketed for erections and exercise.
L-arginine is the direct building block for nitric oxide. A 2019 meta-analysis of 10 randomized trials (540 men) suggested it may help mild to moderate ED at 1,500 to 5,000 mg a day. L-citrulline is converted to arginine in the body and is often promoted as a steadier way to raise arginine, but its ED evidence is thinner: one small pilot study in 24 men with mild ED found 50% reported harder erections on 1.5 g a day for a month, versus 8% on placebo, in a single-blind design. Neither has been shown to match prescription ED drugs.
L-arginine can lower blood pressure further in people taking nitrates (such as nitroglycerin), sildenafil or tadalafil, or blood pressure medicines, and Mayo Clinic advises against it after a recent heart attack. It can also interact with diabetes medicines, blood thinners and potassium-sparing diuretics, and can cause nausea, bloating or diarrhea. Persistent ED can be an early sign of heart disease, so it's worth seeing a doctor rather than self-treating.
VigRX Nitric Oxide is described as a mixture of L-citrulline and L-arginine, but we couldn't confirm the doses or serving size, so we can't tell whether it reaches the amounts used in studies. The product itself has not been tested.