Abstract
Introduction: Substance use disorder (SUD) is a significant global clinical issue marked by the excessive consumption of alcohol, nicotine, and various psychoactive substances, leading to impaired social, cognitive, and occupational functioning. Individuals with SUD frequently experience depression and anxiety disorders, which exacerbate their prognosis and contribute to substantial health and social burdens. The pathophysiology of SUD and its associated conditions is multifaceted, involving multiple dysfunctions in the brain. This complexity underscores an urgent need for the development of noninvasive treatments that can directly target the brain. One of them is transcranial direct current stimulation (tDCS), an intensively studied technique for safely modulating cortical excitability. The aim of this study is to investigate the effectiveness of tDCS in treating symptoms of depression and anxiety in SUD. Methods: With an emphasis on the underlying mechanisms of action, this mechanistic review investigates the effectiveness of tDCS in treating anxiety and depression in SUD patients. Literature searches were conducted using the PubMed/Medline, ResearchGate, Cochrane, and Google Scholar databases. Results: The review identified 12 relevant studies. The results showed that left dorsolateral prefrontal cortex (DLPFC) stimulation is an effective treatment option for depression in SUD. In anxiety disorders, left and right DLPFC stimulation is effective, with better results observed with right DLPFC stimulation. However, the included studies differed in their methodology, sample characteristics, and measurement methods, which could have influenced the final results of the analysis. The central focus of this mechanistic review is to discuss the potential mechanisms of action of tDCS in treating depression and anxiety in SUD. These mechanisms include the modulation of brain networks, a reduction in neuroinflammation, an enhancement in neuroplasticity, and an increase in P300 amplitude. We also discuss the limitations of the included studies and propose ways to address them in future research. Conclusions: This review provides evidence that tDCS is an effective treatment option for anxiety and depression in SUD. Stimulation of the left DLPFC reduces symptoms of depression, while stimulation of the right DLPFC reduces symptoms of anxiety. However, future research is required to confirm these findings and to deepen our understanding of the mechanisms through which tDCS exerts its effects in this context. Neuroimaging methods (fMRI and EEG) and blood tests could be particularly useful.