Sera from patients with PD (n = 20) and healthy volunteers (n = 20) were subjected to the P450 inhibition assay, and the inhibition rates of reactions associated with 12 human P450s (CYP1A1, CYP1A2, CYP2A13, CYP2B6, CYP2C8, CYP2C9, CYP2C18, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5) were determined. This evidence concerns the gene CYP2C19 and Parkinson disease.