Patients who participated in the studies showing negative results [19,20] were not in methadone treatment and, in the case of the Chinese individuals, rarely used illicit non-opioid substances. However, the absence of these two features does not explain the results obtained: the reason is that in China, there was little or no availability of illicit non-opioid substances in the 1990s, and that in both China and Germany, accessibility to methadone treatment was low or absent during this period. In any case, the negative results reported by [19] could also be related with the low severity of the SUD assessed, because this research group examined associations with opioid abuse, rather than opioid dependence, as did the remaining groups.
