Extra Strength Healthy Immune System Function Supports Protein Metabolism Non-GMO A Dietary Supplement Vegetarian - Vegan GMP Quality Assured Citrulline is a non-essential amino acid that is an important intermediate in the urea cycle, functioning along with other amino acids to rid the body of ammonia, a byproduct of protein metabolism. Citrulline also plays an important role in the healing process and in the maintenance of a healthy immune system.