Arginine, also called L-arginine, is an amino acid used by the body as a protein building block. It’s also an intermediate metabolite in the urea cycle and the nitric oxide cycle.^[1][2][3] The urea cycle removes excess ammonia, a toxic molecule, from the body, and the nitric oxide cycle produces nitric oxide, which is an important signaling molecule involved in vascular function.^[1][2][3] Although the body can synthesize arginine, arginine can also be obtained in the diet through protein-containing foods or dietary supplements.^[1][2][3]

Daily supplementation with arginine can reduce blood pressure in people with healthy blood pressure and people with hypertension (high blood pressure).^[4] Evidence also supports the use of arginine to help improve birth outcomes in pregnant women who have conditions like hypertension and preeclampsia, or who have a history of poor pregnancy outcomes. The improved outcomes include a reduced risk of small for gestational age infants^[5] and a reduced risk of intrauterine growth restriction of the baby.^[6] Arginine can also improve maternal outcomes by reducing the risk of preeclampsia in the pregnant mother.^[7][8][9] Additionally, arginine can reduce blood triglycerides,^[10][11] particularly in people older than 50 or in people with metabolic syndrome or type 2 diabetes.^[10]

In men with erectile dysfunction, daily supplementation with arginine can alleviate symptoms.^[12] Furthermore, the combination of arginine with phosphodiesterase type 5 inhibitors (PDE5Is), like Viagra, improves sexual function in men with erectile dysfunction more than treatment with PDE5Is alone.^[13]

Evidence also shows that daily supplementation with arginine might increase VO₂max, but the magnitude of improvement is negligible and unlikely to be clinically meaningful.^[14] Furthermore, most studies find no beneficial effect of arginine on exercise performance.^[15] It is only when highly-varied types of exercise (aerobic, anaerobic, resistance, etc.) and performance outcomes (time-to-exhaustion, time-trials, sprint time, 1-rep-max, reps-to-failure, etc.) are inappropriately pooled that meta-analyses find a performance benefit from arginine.^[15]

Animal studies show that neither oral nor intravenous delivery of arginine causes toxicity, even at high doses.^[16][17] However, supplementation with arginine has been reported to cause adverse gastrointestinal effects in humans, including nausea, vomiting, and diarrhea.^[18] For this reason, upper-limit guidelines of approximately 20 to 30 grams per day have been proposed for supplemental arginine intake,^[19][20] but more long-term safety data in humans are needed to accurately define a tolerable upper intake level.

Nitric oxide is a key molecule involved in vasodilation (the relaxation and widening of blood vessels to increase blood flow to tissues that lack oxygen or nutrients).^[21][2] The beneficial effects of arginine on high blood pressure^[4][7][8] and erectile dysfunction^[12][13] are likely related to vasodilation because arginine increases nitric oxide synthesis.^[21][2][3]

A similar mechanism may explain how arginine might increase VO₂max.^[22][23][14] Additionally, nitric oxide might have direct effects on muscle contractile function.^[22][24] Furthermore, arginine’s role in the urea cycle helps with the excretion of ammonia^[1], the accumulation of which might induce fatigue during exercise.^[25][26][27]