5-HTP functions as the precursor for serotonin, a monoamine neurotransmitter found in the central nervous system (CNS) and some cells of the digestive tract. In the body, 5-HTP is formed by the addition of a hydroxyl group (–OH) to tryptophan, a reaction catalyzed by the enzyme tryptophan hydroxylase. This enzyme can be hindered by stress and conditions such as vitamin B6 deficiency, which is one reason B6 is included in this formula. Additionally, pyridoxal phosphate (vitamin B6) is necessary for 5-HTP to be converted to serotonin. Vitamin C participates in the conversion of tryptophan to 5-HTP and of tyrosine to norepinephrine, another important neurotransmitter. Synthesis of serotonin in the brain requires an adequate supply of either tryptophan or 5-HTP as precursors in the CNS. The supply of tryptophan available for conversion to 5-HTP depends on a number of factors, including the nutritional status of the individual and the competition between tryptophan and other amino acids for transport across the blood-brain barrier. Whereas tryptophan may be sidetracked into the production of niacin or protein, 5-HTP readily crosses the blood-brain barrier and becomes available for serotonin synthesis. Therefore, supplementation with 5-HTP is a more direct route to serotonin production.