Consistent with the existence of a general heritable liability to ASB, having one externalising disorder increases the risk of having other externalising disorders (Krueger, Markon, Patrick, & Iacono, 2005). Use of several drug classes amongst dependent heroin users is common, and is associated with poorer mental health (Darke & Ross, 1997), increased fatal and non-fatal overdose (Darke, Ross, & Hall, 1996; Zador, Sunjic, & Darke, 1996) risky injecting behaviour (Klee, Faugier, Hayes, Boulton, & Morris, 1990) and poorer treatment outcomes (Marsden, et al., 2009). Amongst drug users in treatment, polydrug use is associated with adverse family histories, self-harm, aggression, psychoticism and impulsivity (Martinotti, et al., 2009). ‘Polydrug use’ is used in the research literature to describe the use of two or more drugs within a particular timeframe (concurrent, sequential, 30 day, 12-month or lifetime). The term ‘polysubstance dependence’ is defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) as a substance use disorder where an individual uses at least three different classes of substances repeatedly within a 12-month period, but no single substance predominates (American Psychological Association, 1994).
