5-HTP SynergyA combined formula of 5 hydroxytryptophan and vitamin B6 5 Hydroxytryptophan (5 HTP) is a precursor of serotonin biosynthesis. It is synthesized from L tryptophan, an essential amino acid found naturally in food proteins such as turkey and milk, and requires vitamin B6 as a cofactor. 5 HTP is produced primarily by enterochromaffin cells in the gastrointestinal tract as well as raphe nuclei in the brainstem where it is then converted to serotonin.