5-Hydroxytryptophan or 5-HTP is a naturally occurring amino acid, a precursor to the neurotransmitter serotonin and an intermediate in tryptophan metabolism. It is marketed in the United States and other countries as a dietary supplement for use as an antidepressant, appetite suppressant, and sleep aid. However, according to a 2001 meta-analysis, insufficient high quality research has been done to establish that it is efficacious. The psychoactive action of 5-HTP is thought to derive from its effect on serotonin synthesis in central nervous system tissue. It is believed that an artificially high supply of 5-HTP causes the brain's serotonin-producing neurons to increase production. Increased serotonin production then leads to increased serotonin release. Research shows that co-administration with carbidopa greatly increases plasma 5-HTP levels.However, several studies have reported that 5-HTP is effective even without a peripheral decarboxylase inhibitor (e.g. carbidopa).Other studies have indicated the risk of a scleroderma-like condition resulting from the combination of 5-HTP and carbidopa. 5-HTP, which is found in minute amounts in certain foods like cheese and the white meat of poultry,is often sold as an over-the-counter therapeutic supplement. In this case, it is usually sourced from the seeds of Griffonia simplicifolia. 5-HTP in supplement form is typically sold in 50 mg or 100 mg gelatin or vegetarian capsules.