Serious mental illness and outcomes in inpatient detoxification and rehabilitation: A retrospective cohort study

Background Substance use disorders frequently co-occur with severe mental illness (SMI), and individuals with co-occurring diagnoses often experience greater clinical complexity and poorer treatment outcomes. However, little is known about treatment engagement and outcomes among patients with SMI receiving inpatient addiction treatment in detoxification and rehabilitation settings.Methods This retrospective cohort study examined patients admitted to an inpatient detoxification and rehabilitation unit at an urban academic behavioral health center between January 1 and March 31, 2024. SMI was defined as a history of a psychotic disorder, bipolar disorder, or prior psychiatric hospitalization. Sociodemographic characteristics, clinical history, treatment course, and post-discharge outcomes were extracted from the electronic medical record. Groups with and without SMI were compared using chi-square and independent-samples t-tests. Time to readmission was evaluated using Kaplan-Meier survival analysis. Multivariable logistic regression examined associations with readmission within one year.Results Among 198 patients, 90 (45.5%) met criteria for SMI. Compared with those without SMI, patients with SMI had higher rates of trauma, violence history, unemployment, and prior detoxification or rehabilitation admissions. Despite these differences, there were no significant group differences in detoxification completion, transition to rehabilitation, rehabilitation completion, or length of stay. Readmission rates within one year did not differ between groups in bivariable analyses. In the primary adjusted analysis, SMI was associated with lower odds of readmission (OR = 0.49, 95% CI 0.25-0.98), while greater prior detoxification or rehabilitation utilization was associated with increased readmission risk (OR = 1.04 per admission). The association between SMI and readmission was attenuated in sensitivity analyses.Conclusions Patients with SMI admitted to inpatient detoxification and rehabilitation demonstrated greater psychosocial complexity but similar treatment engagement compared with patients without SMI. These findings suggest that structured inpatient addiction treatment settings may support comparable engagement in care among individuals with co-occurring severe psychiatric illness.