Regarding specific studies on opioid use disorders, our findings agree with two previous reports from Australia [17] and Iran [18], and disagree with two other reports from China [19,20] and Germany [20]. In the Australian study, patients were in methadone treatment and often used non-opioid substances, as in the present study. Both features could maximise a study's ability to detect genetic associations: with non-opioid substance use, because it is also related with the DRD2 A1 allele [16]; and with methadone treatment, because it can be a marker of heroin dependence severity [25].
