Citrulline (L-Citrulline) is a non-essential amino acid that plays an important role in nitrogen metabolism and vascular function. Although it is not directly involved in protein synthesis, citrulline is a key intermediate in the urea cycle and functions as a precursor to L-arginine, indirectly affecting the production of nitric oxide (NO). At the biochemical level, citrulline is converted in the kidneys to L-arginine, increasing its plasma levels more effectively than oral arginine. Increased arginine availability leads to enhanced NO production via the enzyme nitric oxide synthase, resulting in improved vasodilation, blood flow, and delivery of oxygen and nutrients to muscle tissues. Citrulline’s participation in the urea cycle helps to remove ammonia, a metabolic byproduct that accumulates during intense exercise and is associated with fatigue. Through this mechanism, citrulline helps to improve endurance and reduce premature muscle fatigue. From an ergophysiological perspective, citrulline supplementation has been associated with improved muscle perfusion, increased endurance, reduced fatigue, and faster recovery after exercise. In addition, improved performance in high-intensity workouts has been reported, as well as an enhanced feeling of muscle fullness due to increased blood flow. Overall, citrulline is a scientifically documented and widely used amino acid supplement, with the main characteristics of supporting vascular function, improving endurance, and reducing muscle fatigue. It is mainly used in strength and endurance athletes, as well as in individuals seeking to improve physical performance and recovery through better blood circulation and metabolic support.