LAUSR

Abstract Introduction Substance use disorder and posttraumatic stress disorder (PTSD) are highly prevalent among individuals who experience homelessness. However, evaluations of interventions that combine housing and mental health services have reported inconsistent mental health and substance use outcomes when compared to usual services. We investigated 12‐month change in substance use severity and PTSD symptom severity among adults experiencing chronic homelessness and tested whether observed differences were associated with housing, support from mental health services or the Journey to Social Inclusion (J2SI) program. Methods A randomised controlled trial compared the J2SI program with standard service provision (N = 135). Secondary analyses compared those who obtained housing or received mental health services with those who did not. Primary outcomes were alcohol and illicit substance use severity (alcohol, smoking and substance involvement screening test) and PTSD symptom severity (six‐item PTSD checklist). Results There was significant improvement at 12 months in alcohol use severity, illicit substance use severity and PTSD symptoms in the overall sample. Having seen a mental health professional in the previous 12 months was associated with a significant reduction in alcohol and illicit substance use severity but was not associated with changes in PTSD symptom severity. Being housed at 12 months was associated with significantly higher alcohol use severity. Discussion and Conclusions Findings highlight the importance of access to mental health care for people with a history of chronic homelessness. Research is needed to develop and test therapeutic and housing approaches to reduce PTSD symptom severity among people with experience of homelessness.