What is L-arginine? It is an amino acid that may IMPROVE CARDIOVASCULAR HEALTH. Since L-arginine changes into nitric oxide (NO), which causes the blood vessels to dilate and relax it is also a great preworkout supplement. It is a necessary amino acid for cardiovascular health. Scientists have discovered that L-arginine is a NITRIC OXIDE BOOSTER and that without this amino acid there wouldn’t be any nitric oxide.