Nitric oxide (NO) is commonly included in supplements aimed at athletic performance and exercise enhancement, primarily in the form of precursors like L-arginine or L-citrulline. The rationale for its use is based on its role as a vasodilator—NO relaxes the inner muscles of blood vessels, leading to increased blood flow and, theoretically, improved delivery of oxygen and nutrients to exercising muscles. This mechanism suggests a potential benefit for endurance, recovery, and overall exercise performance.

Scientific studies on NO boosters have produced mixed results. Some research has shown that supplementation with L-citrulline or L-arginine can modestly improve exercise performance, particularly in untrained or moderately trained individuals. For example, a few randomized controlled trials have found small improvements in aerobic and anaerobic performance, muscle oxygenation, and reduced fatigue. However, other studies report minimal or no benefit, especially in well-trained athletes. The inconsistency may be due to differences in study design, dosages, populations, and the bioavailability of NO precursors.

Overall, there is a moderate level of scientific evidence supporting the use of NO-related supplements for athletic and exercise aids. The effects, when present, are usually modest and may not be significant for all individuals or types of exercise. The evidence is stronger for acute effects on blood flow than for long-term performance gains.