One limitation that certainly may have affected this study's estimates is the level of survey participation and nonresponse, which varied across countries. This is a likely source of underestimation of illegal drug involvement (and perhaps alcohol and tobacco use), but not necessarily a source of bias with respect to estimated associations with other variables [42]. In the WMHS initiative, we compensated for survey nonresponse via poststratification adjustments, but this approach is limited if nonresponse is associated with drug use in other ways.
