The efficacy and safety of transcranial direct current stimulation in patients with ADHD: a systematic review and meta-analysis

Objective: This meta-analysis evaluated the efficacy and safety of transcranial direct current stimulation (tDCS) for treating Attention-Deficit/Hyperactivity Disorder (ADHD). Methods: Following PRISMA guidelines, we analyzed 28 randomized controlled trials (RCTs) involving 1,864 participants. Outcomes encompassed core ADHD symptoms, hot and cold executive functions (EFs)-including inhibitory control, working memory, and cognitive flexibility-as well as safety profiles based on adverse events. A multilevel meta-analysis was performed using a random-effects model. Subgroup analyses and meta-regressions were conducted to explore potential moderating factors. Results: Compared to sham stimulation, tDCS did not significantly improve core ADHD symptoms (standardized mean difference (SMD) = -0.29, 95% CI -0.59, 0.01, p= 0.05). Similarly, no significant overall effects were observed for cold EFs: inhibitory control (Hedges' g(g)= -0.11, 95% CI -0.26, 0.05, p=0.19), working memory (g= 0.13, 95% CI -0.06, 0.32, p= 0.26), or cognitive flexibility (SMD = -0.42, 95% CI -1.13, 0.29, p= 0.24). The effect on hot EFs was also non-significant (g = 0.27, 95% CI -0.14, 0.70, p = 0.19). Exploratory analyses indicated that anode placement at Fp2 was associated with improvement in both inhibitory control (g= -0.52, 95% CI -0.93, -0.11, p=0.01) and working memory (g = 0.72, 95% CI 0.22, 1.22, p = 0.004), although the overall test for interaction was not significant for inhibitory control (p= 0.19). The most common adverse reactions were mild and transient local skin symptoms, such as itching and redness (RR = 1.42, p=0.04). Conclusion: tDCS was well-tolerated but did not demonstrate significant overall efficacy for core ADHD symptoms or executive functions. Anodal stimulation at Fp2 showed potential selective benefits warranting further investigation. tDCS is not currently recommended as a standalone treatment for ADHD. Future research should optimize stimulation protocols and explore combined interventions with behavioral or cognitive therapies. Systematic Review Registration: https://www.crd.york.ac.uk/PROSPERO, identifier CRD42024612055.