Background: Transcranial Direct Current Stimulation (tDCS) has emerged as a promising non-pharmacological intervention for Attention-Deficit/Hyperactivity Disorder (ADHD). While substantial evidence supports its efficacy in pediatric populations, therapeutic effects in adults remain insufficiently characterized despite the growing recognition of ADHD as a lifelong neurodevelopmental condition with persistent functional impairments.

Methods: To synthesize current evidence regarding the clinical efficacy, cognitive effects, neurophysiological mechanisms, tolerability, and optimal stimulation parameters, 6 electronic databases were systematically searched to identify peer-reviewed controlled trials evaluating tDCS in adults with clinically diagnosed ADHD through May 2025. Data extraction encompassed clinical symptom outcomes, cognitive task performance, neurophysiological findings, adverse events, medication status, and stimulation protocols. Study quality was assessed using the Downs and Black checklist. Exploratory meta-analyses were conducted where appropriate, supplemented by narrative synthesis.

Results: Eleven studies comprising 415 participants met inclusion criteria. An exploratory meta-analysis revealed a significant, moderate-to-large effect of tDCS on inattention symptoms (Hedges' g = –1.62, p = .037), with no corresponding benefit for hyperactivity/impulsivity. Cognitive outcomes demonstrated domain-specific improvements, with inhibitory control and working memory tasks showing the most consistent benefits. Two neurophysiological studies reported tDCS-induced modulation of frontal oscillations associated with cognitive control. Stimulation protocols varied substantially across studies, with no systematic association between specific parameters and efficacy. Treatment durability appeared limited based on the single follow-up study. Adverse events were mild and transient. Critical moderating factors were inconsistently reported and rarely analyzed.

Conclusions: These findings provide preliminary evidence supporting the efficacy of tDCS for inattention symptoms and specific cognitive domains in adults with ADHD, particularly those mediated by prefrontal cortical networks. However, substantial methodological heterogeneity, limited sample sizes, and sparse long-term data constrain definitive conclusions. Future research priorities include protocol standardization, adequately powered trials with extended follow-up periods, integration of neurophysiological biomarkers, and systematic evaluation of individual difference factors to advance precision neuromodulation approaches for adult ADHD.

Tianran Zhang is a PhD researcher at Atlantic Technological University, Ireland. She has published three first-author peer-reviewed articles and co-authored several others in clinical and cognitive neuroscience. Tianran has participated in or coordinated over six interdisciplinary research projects. Her work bridges neurotechnology, digital mental health, and applied psychological research.