L-Arginine is the direct precursor to nitric oxide (NO) via nitric oxide synthase. Our research shows moderate evidence for blood pressure reduction (5.4/2.7 mmHg in meta-analysis) and erectile dysfunction improvement, but for exercise performance, citrulline is now considered superior due to arginine's poor oral bioavailability (~20% absorbed). The key safety concern: arginine is CONTRAINDICATED post-heart attack — the VINTAGE MI trial was stopped early due to increased mortality in the arginine group.
Arginine is converted to nitric oxide by nitric oxide synthase (NOS). NO relaxes blood vessel smooth muscle, lowering blood pressure and improving blood flow — including to the penis (ED mechanism) and muscles (exercise). The problem: oral arginine is heavily metabolized by the enzyme arginase in the gut wall and liver before it reaches the bloodstream. This is why citrulline — which bypasses arginase and converts to arginine in the kidneys — actually raises blood arginine levels MORE than arginine itself.
Reviewed by the Scan Dose Research Team and Clinical Advisory Board | Last updated:
Not medical advice. Based on published clinical research and systematic reviews.