TYR and nervous system disorder: Not limited to the decreased levels of TRP in our previous studies [13,14], the current work expanded the discovery of the alterations in TRP metabolism involving the kynurenic, serotonergic, and microbial pathways in METH abusers, which provided a more detailed analysis of how METH disturbs NT metabolism via TRP- and TYR-associated pathways, which may offer clues of METH-induced psychiatric and neurological disorders.