Supplementary Material for: Kratom (Mitragyna speciosa) Use and Mental Health: A Systematic Review and Multilevel Meta-Analysis

Introduction: Kratom (Mitragyna speciosa) is a medicinal tree native to Southeast Asia. The present multilevel meta-analysis describes the association between kratom use and the positive and negative indicators of mental health. Methods: A total of thirty-six articles were included in the meta-analysis to examine the associations, using a random-effects model. Results: The pooled effect size showed a very small positive association between kratom use and negative indicators of mental health (r=0.092, 95%CI=[0.020, 0.164], p<0.05), while no significant association was found with positive indicators of mental health (r=-0.031, 95%CI=[-0.149, 0.087], p>0.05). Pooled effect sizes of specific mental health outcomes indicated that kratom use showed only a small positive correlation with externalizing disorders (r=0.201, 95%CI=[0.107, 0.300], p<0.001). No significant association was found between kratom use and quality of life (r=0.069, 95%CI=[-0.104, 0.242], p>0.05) and internalizing disorders (r=-0.001, 95%CI=[-0.115, 0.095], p>0.05). Multilevel moderator analysis showed that the pooled effect size of the association between kratom use and substance use disorder (SUD) was stronger in Malaysia (r=0.347, 95%CI=[0.209, 0.516], p<0.001), and with the mean age (β1=-0.035, 95%CI=[-0.055, -0.014], p=0.003), and the drug profile of those who were not co-using other drugs (r=0.347, 95%CI=[0.209, 0.516], p<0.001). Conclusion: The meta-analysis supports the kratom instrumentalization concept, in that a positive gain from kratom consumption can be achieved without any significant adverse associations with mental health.