**1.1 Substance abuse within mental disorders**

The use of psychoactive substances affects mental function at both the short and long- term basis, hence the treatment of substance abuse disorders by mental health professionals. Moreover, literature also shows a bilateral association between the use of psychoactive substances and mental disorders [4], which implies that not only is mental illness a risk factor for later substance abuse [5], but substance abuse is also a risk factor for the development of mental illness [6]. This makes interventions for the prevention of substance dependence an important secondary outcome of interventions for early-onset mental disorders. For purposes of this chapter, reference to mental disorders includes substance abuse.
