IL6 and major depressive disorder: Key measures included psychological status [assessed via the Montreal Cognitive Assessment (MoCA), Self-rating Depression Scale (SDS), Symptom Checklist-90 (SCL-90), and Pittsburgh Sleep Quality Index (PSQI)]; physiological indicators [blood concentrations of serotonin (5-HT), interleukin-6 (IL-6), gamma-aminobutyric acid (GABA), and dopamine (DA)]; and socio-demographic and drug-use characteristics (age, education, marital status, years of drug use, etc.).