Mental health predictors of treatment engagement and completion in substance use disorder treatment: Within- and between-person evidence from a 15-year Western Australian cohort.

INTRODUCTION

Treatment retention remains a major barrier in substance-use disorder (SUD) treatment. Most prior studies focus on single episodes and between-person differences, leaving uncertain how within-person fluctuations in symptom severity and risk indicators affect engagement and completion across repeated episodes. We examined whether baseline and within-episode changes in substance-use severity, psychological distress, and clinical risk profiles predict treatment attendance, duration, and completion across multiple treatment episodes.

METHODS

Using a retrospective cohort design, we analysed administrative data from a Western Australia SUD treatment provider, including 9160 clients, 49,115 episodes and 420,637 appointments between 2009 and 2024. Multilevel Bayesian structural-equation models disaggregated within-person (episode-to-episode) and between-person effects of psychometric measures of substance use, mental health, and clinical risk on treatment completion, with session attendance and episode duration as mediators. Inferential subsamples ranged from 152 to 1914 clients, reflecting site-level assessment-administration practices.

RESULTS

Elevated stress (within and between persons) was consistently associated with poorer attendance and lower completion. Higher suicide risk also reduced attendance and completion, while episode-level increases in depressive symptoms paradoxically increased attendance and, indirectly, completion. Other clinical-risk and substance-use indicators showed weaker or less consistent associations after adjustment.

CONCLUSIONS

Psychological distress and clinical-risk profiles influence treatment completion primarily via engagement behaviours, with within-person effects being robust across sensitivity analyses. Identifying and supporting clients during periods of elevated stress, and sustaining engagement among those with elevated suicide risk, may improve treatment completion.