Objective
Medication non-adherence is frequently reported in patients with major depressive disorder (MDD). The objective of this review is to consolidate data on the prevalence of non-adherence to antidepressant in MDD.

Methods
A systematic review with meta-analysis was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline and the protocol was registered in PROSPERO under the number CRD42021199987. Studies assessing medication adherence in MDD were searched in PubMed/Medline, Embase, CINAHL (The Cumulative Index to Nursing and Allied Health Literature) and PsycINFO. The data extraction was performed by two independents authors. Meta-analysis used random effects model and performed a subgroup analysis.

Results
From the articles retrieved, eleven studies were considered eligible for the final analysis. Most of them assessed non-adherence by self-report scales, followed by Pharmacy Dispensation Records, Monitoring Events Medication System (MEMS) and blood tests. The pooled proportion of non-adherence was 42% (95% IC 30%-54%), but heterogeneity was very large (I2=99%).

Conclusion
Data from the selected studies suggests that a high number of individuals with MDD do not adequately take their medication as prescribed. The high heterogenicity of measures used for the assessment of adherence may have impacted the great variability of the results. The results suggest it is necessary that health care professionals should address this issue in order to achieve a better treatment outcome in major depression.