Neuropsychological functioning and chronic methadone use: A systematic review and meta-analysis.

INTRODUCTION: There is a presumption that neurocognition is commonly impaired in chronic methadone exposed individuals (CM) when compared with healthy controls (HP). Additionally, it remains unclear if short term (<1year) abstinence (AP) is associated with an altered cognitive profile when compared with CM.

METHOD: A random effect model approach was used on data assembled into the Comprehensive Meta-Analysis programme. Cohen's d effect sizes and a significance levels of p<0.01 were calculated for each domain.

RESULTS: Data from a total cohort of 1063 CM, 412 AP and 879 HP participants, from 23 independent studies indicate global impairments in neurocognitive function in CM relative to HP participants. The smaller body of evidence comparing CM to AP participants is inconclusive.

CONCLUSION: Methodological issues such as small sample sizes, heterogeneity and poor quality limited the interpretation of the results and does not address whether the observed impairments reflect co-morbid functioning, methadone-related sedation and/or other factors. Only higher quality longitudinal studies will permit confident interpretation of the results observed in this meta-analysis.