The high prevalence of CD in the study sample also limits our ability to generalize the findings to other OD samples without such comorbidity. Because the SSADDA does not provide details on the specific kinds of opioids that subjects used, there may be other subtypes of OD that are not captured in this study. Independent replication of our findings in a different sample is needed, as are studies using this approach to categorize other substance use and psychiatric features to yield homogeneous subtypes of other disorders. Such subtypes may have utility for gene finding and for clinical characterization and treatment selection. Despite the expectation that the identification of highly heritable subtypes of opioid use and related behaviors will enhance gene-finding efforts, this assumption must be tested empirically.
