This study provides preliminary evidence that the proposed DSM-V opioid use disorder distinction between moderate and severe dependence is valid. Further, the classes are clearly distinguished from each other by other externalizing disorders and multiple opioid overdoses. The association between opioid dependence and other externalizing disorders is consistent with the occurrence of a range of externalizing problems along a continuum [10, 11, 50-52]. As such, increasing severity of one externalizing disorder increases the chances of having other externalizing disorders [11]. The more severe class 1 had an increased risk of meeting criteria for sedative and cocaine dependence, and antisocial personality disorder, as well as experiencing multiple non-fatal opioid overdoses. Removing cocaine and sedative dependence from the model did result in cannabis dependence but not alcohol, nicotine or stimulant dependence, becoming significantly associated with class membership. However, in the final model cannabis dependence did not differ with respect to class membership, perhaps suggesting that cocaine and sedative dependence are better indicators of more severe polydrug dependence. The inclusion of the covariates demonstrates the usefulness of factor mixture modeling, as several covariates
