BACKGROUND: The COVID-19 pandemic led to an unprecedented relaxation of restrictions on take-home doses in opioid agonist treatment (OAT). We conducted a mixed methods systematic review to explore the impact of these changes on program effectiveness and client experiences in OAT. METHODS: The protocol for this review was registered in PROSPERO (CRD42022352310). From Aug.-Nov. 2022, we searched Medline, Embase, CINAHL, PsycInfo, Web of Science, Cochrane Register of Controlled Trials, and the grey literature. We included studies reporting quantitative measures of retention in treatment, illicit substance use, overdose, client health, quality of life, or treatment satisfaction or using qualitative methods to examine client experiences with take-home doses during the pandemic. We critically appraised studies using the Mixed Methods Appraisal Tool. We synthesized quantitative data using vote-counting by direction of effect and presented the results in harvest plots. Qualitative data were analyzed using thematic synthesis. We used a convergent segregated approach to integrate quantitative and qualitative findings. RESULTS: Forty studies were included. Most were from North America (23/40) or the United Kingdom (9/40). The quantitative synthesis was limited by potential for confounding, but suggested an association between take-home doses and increased retention in treatment. There was no evidence of an association between take-home doses and illicit substance use or overdose. Qualitative findings indicated that take-home doses reduced clients' exposure to unregulated substances and stigma and minimized work/treatment conflicts. Though some clients reported challenges with managing their medication, the dominant narrative was one of appreciation, reduced anxiety, and a renewed sense of agency and identity. The integrated analysis suggested reduced treatment burden as an explanation for improved retention and revealed variation in individual relationships between take-home doses and illicit substance use. We identified a critical gap in quantitative measures of patient-important outcomes. CONCLUSION: The relaxation of restrictions on take-home doses was associated with improved client experience and retention in OAT. We found no evidence of an association with illicit substance use or overdose, despite the expansion of take-home doses to previously ineligible groups. Including patient-important outcome measures in policy, program development, and treatment planning is essential to ensuring that decisions around take-home doses accurately reflect their value to clients.