Opioid dependence is frequently associated with polydrug use, mental health disorders, suicidality, overdose, criminal justice system involvement and early onset of substance use and other externalizing behaviors [1-4]. The Diagnostic and Statistical Manual of Mental Disorders (4th edition, DSM-IV) assumes two hierarchically organized categories of opioid use disorder: heroin abuse or heroin dependence [5]. To be diagnosed with opioid dependence, three of seven dependence criteria must be met [5]. Thus, two people with entirely different dependence symptoms may be given the same diagnosis [6]. An important empirical and clinical question is whether or not these differences actually matter. Evidence from the alcohol dependence diagnosis suggests that they do: different patterns of symptoms and levels of severity are likely to be associated with different demographic and clinical characteristics [7-9]. Thus, the current categorical approach to the diagnosis may not reflect what could be a potentially variable expression of the disorder: many disorders, including opioid dependence, may be heterogeneous in both nosologically and clinically meaningful ways. These differences may be important in understanding the course of the disorder and treatment response. Moreover, there is increasing evidence that there is a dimensional aspect associated with many mental disorders [10-13].
