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 effective. Because it is naturally occurring it has not been sought to be approved as a drug in any country.