RECOVERY SUPPORT SERVICES IN SUBSTANCE USE TREATMENT COMPLETION Brett S. Bartruff, University of Pennsylvania Dr. Malitta Engstrom, Dissertation Chair, University of Pennsylvania Dr. James R. McKay, Dissertation Committee Member, University of Pennsylvania Purpose: This study aims to understand the relationship between recovery support services provided by certified peers and clients’ length of time in and completion of substance use treatment. While many innovations in the treatment of substance use have helped improve outcomes for people with substance use disorders, length of time in treatment and completion of treatment continue to be low. Responses to enhance treatment engagement include recovery support programs that are staffed by people with lived experience of substance use and recovery; however, research in this area is limited. Methods: This study involves an archival chart review from a western state community mental health agency in the United States. Data for the study are from the agency’s electronic medical record during a 24-month period between January 1, 2018 and December 31, 2019. The sample (N=1,007) is comprised of clients who engaged in substance use treatment at the agency during the 24-month inclusion period. The study compares people in substance use treatment who chose to receive recovery support peer services (n=216) with people who opted out (n=791). Because there were statistically significant differences between the two groups, a subsample comparison group was created using a one-to-one match from the peer and non-peer groups across four variables (n=408), which included gender, prior treatment, co-occurring psychiatric disorder, and opioid use. To test treatment effects across groups (people who received peer services and the matched no-peer services comparison group), an independent sample t-test was used to test the difference in length of treatment and a chi-square test of independence was used to examine treatment completion. Additionally, to test treatment effects across peer services groups (people who received peer services and the matched no-peer services comparison group) while controlling for demographic and clinical characteristics, two hierarchical regression analyses were used to test the predictive value of peer services on length of stay in treatment and treatment completion. To test treatment effects in the peer-services group, multivariable regression analysis was used to test two models, one for each dependent variable, length of treatment and treatment completion. Results: This study’s initial treatment effect testing finds that people who received any recovery support service stayed in substance use treatment an average of 27 more days than people who did not receive any recovery services. However, the additional hierarchical regression analysis finds that when controlling for demographic and clinical characteristic covariates, there was no statistically significant difference in treatment length between people who received peer services and the matched no-peer services comparison group. Among people who received recovery support services, each additional session with a certified peer worker predicted an 11% increase in the likelihood of completing substance use treatment. No statistically significant difference was found in the likelihood of completing treatment when comparing people who did and did not receive recovery support services. The post-hoc analysis provides additional findings, including the number of different kinds of peer services provided, years of education, and workforce participation as predictors of both dependent variables among people in the peer services group. The number of previous treatment episodes also had a statistically significant positive association with increased days in treatment among people in the peer services group. Implications and Conclusion: As the involvement of peers to assist in substance use treatment grows around the United States, this study’s findings support the promise of this trend. Peer support services may be an important part of enhanced treatment engagement. As substance use treatment professionals and agencies look for ways to improve outcomes with clients, this study provides support for the value of peer support involvement in traditional substance use treatment among people who choose to engage with peer support services.