Among PD patients, users of dopaminergic agents and trihexyphenidyl exhibited lower plasma levels of PAGln, Pcs, Pcg, and IS compared to non-users (all P < 0.05), while users of catechol-O-methyltransferase inhibitor and amantadine had increased levels of PAGln, PAA, Pcs, Pcg, and IS compared to non-users (all P < 0.001) (Supplementary Table 3). This evidence concerns the gene COMT and Parkinson disease.