“…A questionnaire was specifically designed for this study. Because previous population-based studies have reported a significant associations between socio-demographic factors, particularly low socio-economic status (as measured by education and income), and pain [ 31 – 33 ], socio-demographic variables in the questionnaire included gender, age, education years, marital status, employment status, and self-rated financial status (good, fair, poor). Because substance use characteristics (i.e., long-term heroin use and injection drug use) and psychological factors have been found to be significantly associated with pain [ 34 – 36 ], clinical data collected in the study were usual route of past heroin administration (smoking, injecting), duration of past heroin use, duration of MMT, methadone dosage, depression, and anxiety.…”