Tryptophan (IUPAC-IUBMB abbreviation: Trp or W; IUPACabbreviation: L-Trp or D-Trp; sold for medical use as Tryptan) is one of the 22standard amino acids and an essential amino acid in the human diet, asdemonstrated by its growth effects on rats. It is encoded in the standardgenetic code as the codon UGG. Only the L-stereoisomer of tryptophan is used instructural or enzyme proteins, but the R -stereoisomer is occasionally found innaturally produced peptides (for example, the marine venom peptide contryphan).The distinguishing structural characteristic of tryptophan is that it containsan indole functional group.

There isevidence that blood tryptophan levels are unlikely to be altered by changingthe diet, but for some time, tryptophan has been available in health foodstores as a dietary supplement.

Clinicalresearch has shown mixed results with respect to tryptophan's effectiveness asa sleep aid, especially in normal patients. Tryptophan has shown someeffectiveness for treatment of a variety of other conditions typicallyassociated with low serotonin levels in the brain. In particular, tryptophanhas shown some promise as an antidepressant alone and as an"augmenter" of antidepressant drugs. However, the reliability ofthese clinical trials has been questioned because of lack of formal controlsand repeatability. In addition, tryptophan itself may not be useful in thetreatment of depression or other serotonin-dependent moods, but may be usefulin understanding the chemical pathways that will give new research directionsfor pharmaceuticals.