This study has limitations. The first concerns generalizability, as the current sample is largely Caucasian. Further, this sample has a very low alcohol abstinence rate and findings may not generalize to groups with more limited alcohol involvement. Second, in Australia, cannabis-containing joints often include tobacco (Bélanger, Akre, Kuntsche, Gmel, & Suris, 2011), and cannabis and tobacco have a common route of administration. Both of these factors may help explain some of the genetic and environmental overlap between the substances. Thus, cannabis- or tobacco-“specific” estimates may reflect some cross-substance processes. Studies of polysubstance involvement employing samples that do not co-use cannabis with tobacco will be important to examine the replicability of the current findings. Third, we employed a restricted CUD measure. This was necessary to harmonize the Cohort II and Cohort III assessments. Therefore, current results may not generalize to analyses employing complete DSM-IV CUD symptom counts. However, it should be noted that when analyses were re-run in Cohort III using the 11-item symptom count, the results were very comparable to those obtained with the restricted variable. Fourth, Cohorts II and
