5-Hydroxy-L-tryptophan is an aromatic amino acid naturally produced by the body from the essential amino acid L-tryptophan. 5-Hydroxy-L-tryptophan is the immediate precursor of the neurotransmitter serotonin. The conversion to serotonin is catalyzed by the enzyme aromatic L-amino acid decarboxylase (EC 4.1.1.28) (AADC1 also known as DOPA decarboxylase), an essential enzyme in the metabolism of the monoamine neurotransmitters. An accumulation of 5-hydroxy-L-tryptophan in cerebrospinal fluid occurs in aromatic L-amino acid decarboxylase deficiency (AADC deficiency) (OMIM: 608643) accompanied by an increased excretion in the urine of the patients, which are indicative of the disorder but not specific. 5-Hydroxy-L-tryptophan is also increased in other disorders such as in Parkinson's patients with severe postural instability and gait disorders. The amount of endogenous 5-hydroxy-L-tryptophan available for serotonin synthesis depends on the availability of tryptophan and on the activity of various enzymes, especially tryptophan hydroxylase (EC 1.14.16.4), indoleamine 2,3-dioxygenase (EC 1.13.11.52), and tryptophan 2,3-dioxygenase (TDO) (EC 1.13.11.11). 5-Hydroxy-L-tryptophan has been used clinically for over 30 years. In addition to its use in the treatment of depression, the therapeutic administration of 5-hydroxy-L-tryptophan has been shown to be effective in treating a wide variety of conditions, including fibromyalgia, insomnia, binge eating associated with obesity, cerebellar ataxia, and chronic headaches. 5-Hydroxy-L-tryptophan easily crosses the blood-brain barrier and effectively increases central nervous system (CNS) synthesis of serotonin. Supplementation with 5-hydroxy-L-tryptophan is hypothesized to normalize serotonin synthesis, which is putatively related to its antidepressant properties (PMID: 9295177, 17240182, 16023217). When present in sufficiently high levels, 5-hydroxytryptophan can be a neurotoxin and a metabotoxin. A neurotoxin is a compound that disrupts or attacks neural cells or tissue. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. Signs and symptoms of AADC deficiency generally appear in the first year of life. Affected infants may have severe developmental delay, weak muscle tone (hypotonia), muscle stiffness, difficulty moving, and involuntary writhing movements of the limbs (athetosis). They may be lacking in energy (lethargic), feed poorly, startle easily, and have sleep disturbances. Since 5-hydroxytryptophan is a precursor to serotonin, altered levels of serotonin can accumulate in the brain, which leads to abnormal neural signalling. Infants with AADC deficiency have very low levels of neural signalling molecules while individuals who consume high levels of 5-hydroxytryptophan will have very high levels of neural signalling molecules. Both conditions can lead to vomiting, nausea, extreme drowsiness, and lethargy. 5-Hydroxytryptophan (5-HTP), also known as oxitriptan (INN) is sold over-the-counter in the United Kingdom, the United States, and Canada as a dietary supplement for use as an antidepressant, appetite suppressant, and sleep aid. It is also marketed in many European countries for the indication of major depression under trade names such as Cincofarm, Levothym, Levotonine, Oxyfan, Telesol, Tript-OH, and Triptum. Several double-blind placebo-controlled clinical trials have demonstrated the effectiveness of 5-HTP in the treatment of depression, though a lack of high-quality studies has been noted. More and larger studies are needed to determine if 5-HTP is truly effective in treating depression.