Tryptophan is an amino acid precursor of serotonin and melatonin . 6-fluoro-DL-Tryptophan is a serotonin synthesis inhibitor that is metabolized in the brain and may be useful for tracing pools of neuronal serotonin. It is used as a competitive inhibitor of tryptophan binding to albumin and can behave as a substrate for the serotonin neural transporter for passage through the blood brain barrier.