BACKGROUND Mental disorders are common among people with opioid use disorder. This review synthesised evidence of the association of comorbid mental disorders on retention with buprenorphine and methadone treatment.

METHODS A systematic search of PubMed, Embase and PsycInfo was conducted in 2025, using search terms related to opioid use disorder and opioid agonist treatment medications of buprenorphine or methadone. Included studies reported on adults aged ≥ 18 years with opioid use disorder in any treatment setting, measured mental disorders or symptoms of mental disorders and retention in buprenorphine or methadone. Any retention data by mental disorders or symptoms of mental disorders were extracted for analysis.

RESULTS Of 16,056 papers screened, 48 cohorts were included for analyses, consisting of 151,570 individuals. Our meta-analyses indicated that people with mood or personality disorders are more likely to drop out of buprenorphine treatment at 12 and 24 months, compared to people without these specific disorders. At 24 months, people with depression, bipolar disorder, post-traumatic stress disorder, panic disorder and attention-deficit/hyperactivity disorder are more likely to have ceased buprenorphine treatment, compared to people with no mental disorder. People with psychotic and personality disorders were more likely to be retained in methadone treatment at 24 months compared to either people without these specific disorders or with no mental disorders.

CONCLUSION Among people who have opioid use disorder, mental disorders are associated with poorer retention in buprenorphine treatment from 6 months. The small number of cohorts within each analysis emphasises a further need for studies examining the association between mental disorders and buprenorphine or methadone retention. This review highlights the importance of clinicians in assessing for comorbid mental disorders to facilitate appropriate long-term care and improve treatment outcomes.