Relapsing on opioids after residential treatment may involve fatal outcomes, considering the potential for reduced tolerance and the potency of fentanyl in the illicit opioid market. The present paper examines recent literature on the risk of relapse among adults with opioid use disorder after discharge from residential treatment. We searched for published studies from 2018 to 2022 through database searches, including CINAHL, PsychINFO, PubMed, and Scopus. Across the N=10 studies included in this review, returning to substance use after residential treatment was captured differently, including self-report, hair samples, and urine samples. Follow-up relapse data after discharging from treatment was also captured across different time periods of included studies ranging from one month to six months. Variability was also identified in the percentage of individuals who relapsed after treatment, ranging from 0% to 95%. Considering the potential for a fatal overdose in the current fentanyl era, it is imperative to provide resources during residential treatment that can reduce the risk of relapse after discharge.