NOW - L-Arginine - 120 Tablet(s) - NPN: 80002670 -- L-Arginine is a conditionally essential basic amino acid involved primarily in urea metabolism and excretion, as well as DNA synthesis. L-Arginine is the substrate for the synthesis of nitric oxide (NO), a cell-derived relaxing factor that is essential for regulating vascular tone and blood flow. It also supports the manufacture of creatine, an energy substrate for muscles. 95% of creatine is located in the skeletal muscle. This is a double-strength formula.