The relationship between mental health, substance use, and homelessness is complex. An estimated 48% 98% of youth who are homeless meet the criteria for at least one mental health diagnosis (Hodgson, Shelton, van den Bree, & Los, 2013). Common disorders include depressive disorders, anxiety disorders, posttraumatic stress disorder, psychosis, substance use disorders, and attention-deficit/hyperactivity disorder (Hodgson et al., 2013). Between 69% and 86% meet the criteria for a substance use disorder (Medlow, Klineberg, & Steinbeck, 2014). Compared with their housed peers, youth who are homeless have elevated rates of co-occurring substance use and mental health disorders (Hodgson et al., 2013; Kamieniecki, 2001). While mental health disorders and substance misuse can increase the risk of experiencing homelessness, homelessness itself can exacerbate preexisting mental health issues and new psychological symptoms and maladaptive behaviours, such as substance use (Hodgson et al., 2013). Given the limited access that youth who are homeless have to healthy coping mechanisms, substance use may be one of the only ways they have learned to cope with mental health problems and the challenges of homelessness.

Left untreated, substance use and mental health problems create additional barriers to exiting homelessness (Hodgson et al., 2013; Medlow et al., 2014). They can increase vulnerability, putting youth at further risk of street victimization (Bender, Ferguson, Thompson, Komlo, & Pollio, 2010; Whitbeck, Hoyt, & Yoder, 1999). Substance use and mental health problems are also associated with increased deviant behavior, which can lead to arrests and legal trouble (Chen, Thrane, Whitbeck, & Johnson, 2006). They can also decrease motivation to improve the situation (Auerswald & Eyre, 2002). This complacency may decrease the likelihood that youth will seek assistance from social programs, obtain or maintain employment, or take steps toward stabilization (Auerswald & Eyre, 2002). Intervention efforts to improve the lives of these youth may have limited impact if underlying substance use and mental health problems are not treated.