Citrulline is formed from ornithine, ammonia and carbon dioxide. When combined with aspartic acid it forms arginosuccinic acid, and is then converted into arginine after further metabolism. It exists primarily in the liver and is a major component of the urea cycle. It stimulates the immune system, and is used in the treatment of fatigue. It detoxifies ammonia, which in excess amounts can damage cells. As a precursor of both arginine and ornithine, it can influence the production of Growth Hormone.