Efficacy and safety of transcranial direct current stimulation for children and adolescents with attention-deficit/hyperactivity disorder: a systematic review and meta-analysis

Abstract

Background:

Children and adolescents with attention-deficit/hyperactivity disorder (ADHD) often show cognitive deficits. Given that some evidence has suggested transcranial direct current stimulation (tDCS) as a potential alternative or adjunct to psychostimulants, we sought to perform a meta-analysis and systematic review to evaluate the effects of tDCS on clinical symptoms and cognitive function among children and adolescents with ADHD, as well as to summarize associated adverse effects.

Methods:

We searched PubMed, Embase, Web of Science, Scopus, and the Cochrane Library up to May 7, 2025 for randomized controlled trials (RCTs) involving children and adolescents with ADHD who underwent tDCs therapy. The outcome included specific cognitive function assessments and clinical symptoms.

Results:

We included 18 RCTs that involved 496 children and adolescents with ADHD, of which 14 trials (n = 388) were included in the meta-analysis. The results indicated that there was no significant improvement in clinical symptoms (standardized mean difference [SMD] 0.012, 95% confidence interval [CI] −0.235 to 0.259) and processing speed (SMD 0.063, 95% CI −0.145 to 0.27) compared with controls. For cognitive function, those who underwent tDCS showed significant improvement effects in attention (SMD 0.207, 95% CI 0.011 to 0.403) and inhibitory control (SMD 0.222, 95% CI 0.045 to 0.399). Subgroup analyses revealed that stimulation at the F3 site was more effective in improving attention, inhibitory control, and processing speed. A current intensity of 1 mA outperformed currents of 1.5 mA and 2 mA in enhancing inhibitory control, and the cathode was more effective than the anode. A single stimulation session appeared effective in improving attention and inhibitory control, although further studies are needed to confirm these findings.

Limitations:

Some subgroup analyses included few studies, lacked ADHD subtype delineation, and involved only single-dimensional analysis, which limited comprehensive conclusions.

Conclusion:

Overall, tDCS may improve the attention and inhibitory abilities of children and adolescents with ADHD, particularly with optimal stimulation parameters (F3 site, a current intensity of 1 mA, cathodal stimulation, and single-session stimulation). These findings suggest therapeutic potential but require larger clinical validation.

Introduction

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent neurodevelopmental disorder among children and adolescents, with a global prevalence estimated at 5%–10%.1 The disorder is characterized by core symptoms of persistent inattention, hyperactivity, and impulsivity.2 People with ADHD often exhibit cognitive deficits,3 such as impairments in attention,4 working memory,5,6 inhibitory control,7 processing speed,8,9 and executive function.10,11 These symptoms typically persist into adolescence and adulthood, potentially affecting academic achievement12 and social interactions.13

Neuroimaging and neuropsychological studies have shown that distinct ADHD symptoms can be attributed to the functional localization of specific brain regions, particularly the lateral prefrontal cortex and its connectivity with the basal ganglia. Other regions include the medial frontal lobe, cingulate cortex, and orbitofrontal regions, along with dissociable frontoparietal, frontolimbic, and frontocerebellar networks.14,15 Specifically, deficits in executive function are associated with reduced grey matter volume in the right middle temporal gyrus,16 response inhibition is linked to decreased activation in the frontostriatal circuit,17 and motor response inhibition is correlated with insufficient activation in the right inferior frontal cortex (IFC), supplementary motor area, basal ganglia, and thalamus, coupled with heightened activation in the posterior cingulate cortex. Similarly, interference inhibition is associated with insufficient activation in the right IFC, anterior cingulate cortex, basal ganglia, and thalamus, alongside enhanced activation in the anterior medial prefrontal cortex. Attention deficits are related to reduced activation in the right dorsolateral prefrontal cortex (DLPFC), posterior thalamus, inferior parietal lobule, and precuneus, as well as increased activation in the cerebellum and occipital regions.18 Finally, timing tasks exhibit correlations with reduced activation in the left IFC, left inferior parietal lobule, and right cerebellum.19 Several studies have demonstrated that people with ADHD exhibit distinct differences in brain structure compared with healthy controls. Specifically, these differences are evident in both overall intracranial volume and subcortical grey matter volume. People with ADHD tend to have a smaller overall intracranial volume, as well as reduced volumes in key subcortical structures such as the nucleus accumbens, amygdala, caudate, hippocampus, and putamen. 20 A meta-analysis of research on brain structure volumes in patients with ADHD further underscores that the most substantial reductions are observed in subcortical regions, particularly the basal ganglia.21

Treatment approaches for ADHD can be broadly categorized into pharmacological and nonpharmacological interventions. Pharmacological therapy primarily involving stimulant and nonstimulant medications, is considered the most effective short-term treatment for ADHD.22,23 However, these medications are associated with adverse effects and often require long-term administration.24,25 Nonpharmacological therapies can be primarily categorized into behavioural interventions, cognitive training, and neurofeedback training. However, their therapeutic efficacy has been found to be limited, 23,26,27 necessitating the exploration of new complementary treatment options, particularly those grounded in the neuropathological mechanisms of ADHD.

Transcranial direct current stimulation (tDCS) is a painless, noninvasive, and well-tolerated brain stimulation therapy that can serve as an alternative or complementary treatment for various neuropsychiatric disorders. It can also function as an adjunctive or alternative therapy to psychostimulants in ADHD.28 This approach specifically targets key brain regions implicated in ADHD, such as the prefrontal cortex and subcortical frontal systems,18 by delivering a weak, continuous current through scalp electrodes to modulate cortical excitability and induce neuroplasticity changes. The direction of these changes depends on the polarity of the stimulation. Anodal stimulation increases excitability, while cathodal stimulation decreases it.29 However, the effects of tDCS may be influenced by factors such as age, heterogeneity in cognitive measurements, and stimulation protocol (e.g., stimulated sites, current intensity, polarity, duration, number of sessions). 30 Given the ongoing discussion of the therapeutic efficacy of tDCS for ADHD31,32 and the lack of a standardized stimulation protocol, further investigation through systematic reviews or meta-analyses is warranted.

We sought to perform a comprehensive meta-analysis and systematic review of tDCS as a therapeutic intervention for ADHD. Specifically, we investigated its effects on both clinical symptoms and cognitive functions — attention, inhibitory control, and processing speed — to elucidate the effect of tDCS on specific cognitive areas. Moreover, we conducted subgroup analyses by stimulation site, current intensity, electrode polarity, and the number of stimulation sessions to evaluate the efficacy of various stimulation protocols. These analyses were expected to yield valuable insights for future research and clinical applications. Finally, we systematically documented the adverse effects and reactions reported in the literature.

Methods

Search strategy

The initial search adhered to the Population, Intervention, Comparison, Outcome (PICO) framework. We searched electronic databases (PubMed, Embase, Cochrane Library, Web of Science, Scopus) for randomized controlled trials (RCTs) investigating the effects of tDCS on children and adolescents with ADHD. The search period spanned from the inception of each database to May 7, 2025. Only English-language publications were included.

We used the following search terms in combination: “transcranial direct current stimulation,” “tDCS,” “transcranial electric stimulation,” “attention-deficit/hyperactivity disorder,” “ADHD,” “hyperkinetic disorder,” “inattention,” “hyperactivity,” “impulsivity,” and “randomized controlled trial” (Appendix 1, Section S1, available at www.jpn.ca/lookup/doi/10.1503/jpn.250032/tab-related-content, provides the full search strategy). Electronic database searches were complemented by manual searches to identify potentially eligible records.

The review was reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement,33 and followed the methods described in the Cochrane Handbook for Systematic Reviews of Interventions (version 6.5). We registered the protocol in the International Prospective Register of Systematic Reviews (CRD42024567780).

Eligibility criteria

We included RCTs that compared control groups (i.e., placebo [sham], baseline, or waiting list controls) and children and adolescents (aged 3–18 yr) with ADHD (any subtype) based on clinical diagnosis or validated ADHD rating scales or research diagnostic questionnaires (e.g., Conners 3rd Edition Parent). Experimental groups received tDCS while the control group received placebo (sham) or other treatments consistent with the combined therapy of the experimental group. Outcome measures must have included at least 1 domain of cognitive function (attention, inhibitory control, processing speed), clinical symptom assessment of ADHD, and reports of adverse reactions and effects. All reports must have provide data on means and standard deviations.

We excluded non-English language articles, articles for which the full text was not accessible, and studies with unquantified results or lacking specified outcome measures.

Data extraction

We checked the retrieved articles for duplicates using Endnote software. Two reviewers independently evaluated the titles and abstracts of retrieved studies. Full versions of all potentially eligible studies were retrieved. Using a standardized form, 2 independent reviewers extracted baseline characteristics and outcome data from full-text articles, including general information (first author, publication year, country, design type), characteristics of the study participants (sample size, age, sex), intervention protocol details (electrode polarity, stimulation site, current intensity, frequency, duration), intervention measures, outcome measures and assessment tools (priority was given to primary outcomes; if unavailable, secondary outcomes were extracted), and adverse reactions and effects during and after the intervention. Any discrepancies between the 2 reviewers were resolved through discussion, with involvement of a third reviewer if necessary. Despite the unavailability of original data from 3 studies34–36 and the lack of mean and standard deviation reporting in 1 study,37 we included these 4 studies in the systematic review to provide a comprehensive overview of the existing empirical evidence.

Methodological quality assessment

Using the Cochrane Collaboration’s tool,38 we assessed risk of bias in 5 domains: selection bias, performance bias, detection bias, attrition bias, and other biases. Two reviewers (M.Z. and C.M.) independently assessed the risk of bias for all studies, with each domain assessed as low risk of bias, some concerns, or high risk of bias. Disagreements were resolved by discussion or adjudication.

Statistical analysis

We performed all analyses using Stata 15.1. For continuous data, We used Hedges’ g and 95% confidence intervals (CIs) as effect size statistics. Given the variability in scales used across studies to assess the same outcome and the anticipated heterogeneity, we used standardized mean differences (SMDs) and a random-effects model for analysis. We assessed statistical heterogeneity among studies using the χ² test and I² statistic. Heterogeneity was considered significant if p values were less than 0.1 and I² was greater than 50%. For I², 0%–25% indicated low heterogeneity, 25%–50% indicated low-to-moderate heterogeneity, 50%–75% indicated moderate-to-high heterogeneity, and greater than 75% indicated high heterogeneity. We employed subgroup analyses to investigate potential sources of heterogeneity. To assess their effect on study outcomes, we conducted subgroup analyses based on 4 key factors, namely stimulated site (e.g., left DLPFC [F3], right inferior frontal cortex [F8], right inferior frontal [F6] + F8, left anterior frontal [AF3] + F3, F3 + right middle frontal [F4], right posterior parietal cortex [P4], right frontal pole [Fp2]), current intensity (e.g., 1 mA, 1.5 mA, 2 mA), polarity (e.g., anodal, cathodal), and number of stimulation sessions (e.g., 1, 5, 10, 12, 15).

To reduce high heterogeneity in the meta-analysis, we clustered cognitive effects of tDCS into clearly separated cognitive domains of inhibitory control, attention, and processing speed.

Results

The initial search retrieved 957 potentially relevant studies. Among these initial records, we excluded 482 duplicate articles. The review of titles and abstracts left 76 articles for further review. We exlcluded 62 articles as they did not meet the inclusion criteria. We included 4 articles via alternative sources. Consequently, we included 18 studies in this meta-analysis.; 4 articles34–37 lacked original data and were therefore included in the systematic review only (Figure 1).34–37,39–52

Figure 1.

Flow diagram of literature search process. See Related Content tab for accessible version.

Study characteristics

Descriptive characteristics of the included studies are detailed in Table 1 and Table 2. The 18 studies involved a total of 496 participants. The intervention group primarily received tDCS; 3 studies combined tDCS with cognitive training, 39,41,42 and 1 study combined tDCs with video games.44 The control group received sham tDCS. Eighteen studies were published between 2015 and 2025, with a diverse participant population. Seven studies were from Germany, 37,43,46,48,50–52 2 were from the United Kingdom,39,41 3 were from Iran,36,47,49 and the remaining studies were from Italy,35 China,34 Brazil,40 Israel,42 India,44 and Thailand.45 Six studies34,39,41,42,48,52 reported ADHD clinical symptom assessments before and after the intervention. Only 4 studies44,45,47,51 did not report adverse reactions or effects following the intervention. All studies provided results of cognitive function assessments both before and after the intervention.

Table 1.

Participant details from the included studies

Study	Design	No. of participants	Age, yr, mean ± SD (range)	No. of males/females	Intellect, mean ± SD (range)	Comorbidity	Stimulant medication
Westwood et al.30	Double-blind, sham-controlled RCT	Active: 24 Sham: 26	Active: 13.05 ± 1.98 Sham: 14.23 ± 2.06 (10–18)	50/0	IQ Active: 100.08 ± 13.17 Sham: 105.15 ± 13.83	Not ruling out ODD	Abstain from psychostimulants at least 24 h before each assessment time point.
D’Aiello et al.35	Single-blind, sham-controlled RCT	26	10.63 ± 1.41	24/2	IQ 106.42 ± 10.85	–	One of the control group: a single dose of 5–10 mg of immediate-release MPH.
Wang et al.34	Sham-controlled RCT	Active: 24 Sham: 23	Active: 11.29 ± 2.51 Sham: 11.74 ± 2.59	27/20	IQ ≥ 80	–	Patients who had used any medication in the past and recently to treat ADHD were excluded.
Guimarães et al.40	Triple-blind, sham-controlled, crossover RCT	15	11.2 ± 3.0	10/5	IQ 90.3 ± 10.3	–	–
Westwood et al.41	Double-blind, sham-controlled RCT	Active: 10 Sham: 13	Active: 154.3 mo ± 23.03 mo Sham: 174.07 mo ± 22.38 mo	23/0	IQ ≥ 80	Not ruling out CD or ODD	Abstain from psychostimulants at least 24 h before each assessment time point.
Schertz et al.42	Double blind, sham-controlled RCT pilot	Active: 13 Sham: 12	10.83 ± 1.79	18/7	Anford Binet-5 score > 70	–	Abstain from psychostimulants at least 1 mo before and throughout the study period.
Salehinejad et al.43	Single-blinded, sham-controlled, crossover RCT	Active: 11 Sham: 11	8.86 ± 1.80	11/11	–	–	–
Makkar et al.44	Unblinded RCT	Active: 30 Sham: 31	Active: 12.80 ± 1.75 Sham: 12.81 ± 2.01 (10–16)	49/12	–	–	–
Klomjai et al.45	Double-blind, crossover, sham-controlled RCT	Active: 5 Sham: 6	8.55 ± 0.65 (7–14)	10/1	–	–	6 participants were undergoing medication treatment.
Salehinejad et al.46	Single-blinded, sham-controlled, crossover RCT	17	9.53 ± 1.5 (8–12)	12/5	–	–	Refrained from medication (stimulants) at least 24 h before each session.
Salehinejad et al.47	Single-blinded, crossover, sham-controlled RCT	20	(15–17)	–	–	–	–
Soff et al.48	Double-blinded, sham-controlled crossover RCT	15	14.2 ± 1.2 (12–16)	12/3	IQ 99.7 ± 12.5	–	6 participants discontinued medication at least 96 h before the first assessment and were not allowed to use medication throughout the whole study.
Nejati et al.49	Double-blind, sham-controlled, crossover RCT	15	10 ± 2.3	15/0	–	–	Not being on ADHD medication during the experiment.
	Double-blind, sham-controlled, crossover RCT	10	9 ± 1.8	5/5	–	–	Not being on ADHD medication during the experiment.
Sotnikova et al.37	Double-blind, sham-controlled experimental design	13	14.21 ± 1.28	10/3	IQ 99.5 ± 12.15	–	5 patients receiving stimulant treatment discontinued medication at least 96 h before the first assessment.
Breitling et al.50	Single-blind, sham-controlled, crossover RCT	21	14.33	21/0	IQ: Active: 100 Sham: 105.33	1 patient fulfilled diagnostic criteria for CD	Refrained at least 24 h before each experimental session from taking medication.
Munz et al.51	Double-blind, sham-controlled, crossover RCT	14	12.3 ± 1.39 (10–14)	14/0	IQ 102 ± 9.44	ODD (n = 3), CD (n = 2)	10 patients had previously been using MPH but discontinued medication 48 h before each of the experimental conditions.
Krauel et al.52	Sham-controlled, double-blind, parallel-group RCT	Active: 16 Sham: 19	Active: 12.6 (11.7–15.2) Sham: 12.5 (11.0–15.4)	27/8	Nonverbal IQ: Active: 103 (92–115) Sham: 104 (94–112)	ODD, CD, anxiety disorders, elimination disorders	Refrained from medication use during the study.
		Active: 17 Sham: 17	Active: 13.5 (11.7–14.2) Sham: 14.4 (12.2–14.9)	27/7	Nonverbal IQ: Active: 101 (90–111) Sham: 103 (94–115)
Nejati et al.24	Single-blinded, sham-controlled, crossover RCT	22	9.16 ± 1.63	14/8	–	–	The washout period for the medication was 24 h before each stimulation session.

ADHD = attention-deficit/hyperactivity disorder; CD = conduct disorder; IQ = intellectual quotient; MPH = methylphenidate; NA = not applicable; ODD = oppositional defiant disorder; RCT = randomized controlled trial; SD = standard deviation.

Table 2.

Stimulation protocols and outcome indicators of included studies

Study	Stimulation protocol								Outcomes
	Polarity	tDCS montage, target/reference	Current, mA	No. of sessions	Duration, min	Intervention time	Timing	Intervention	Clinical	Cognitive
Westwood et al.30	Anodal	F8/Fp1 (25 cm2 both)	1	15	20	15 consecutive weekdays	Offline	Multi-session anodal-tDCS + CT	ADHD-RS	Go/no-go; CPT
D’Aiello et al.35	Anodal	F3/Fp2 (25 cm)	1	1	20	3 d	Offline	tDCS	–	Go/no-go; N-back
Wang et al.34	Anodal	Fp2 (5 × 0.785 cm2)/Fpz; Afz; AF4; AF8 (4 × 0.785 cm2)	1	10	20	12 d (5 d stimulation + 2 d rest + 5 d stimulation)	Offline	HD-tDCS	SNAP-IV; CPRS	IVA + CPT; Stroop Colour and Word Test; Tower of Hanoi
Guimarães et al.40	Anodal	F3/Fp2 (35 cm2 both)	2	5	30	5 d, consecutive	Offline	tDCS	–	TAVIS-4
Westwood et al.41	Anodal	F8/Fp1 (25 cm2 both)	1	15	20	15 consecutive weekdays	Offline	Multi-session anodal-tDCS + CT	ADHD-RS	Go/no-go; CPT
Schertz et al.42	Anodal	F3/Cz (25 cm2 both)	1	12	20	4 wk (12 sessions)	Offline	Multi-session anodal-tDCS + CT	VADPRS	CANTAB (CPT)
Salehinejad et al.43	Anodal	F3 + F4/contralateral shoulders (16 cm2 both)	1.5	1	15	Once	Online	tDCs	–	N-back; WCST; Go/no-go; Flanker
Makkar et al.44	Anodal	F3/Fp2 (NA)	1	12	20	4 wk (3 times a week)	Online	tDCS + video game	–	RPM; Stroop; TMT
Klomjai et al.45	Cathodal	F3/Fp2 (25 cm2 both)	1.5	5	15	5 d	Offline	tDCS	–	Go/no-go; CPT (P300)
Salehinejad et al.46	Anodal	P4/left shoulder (25 cm2 both)	1	1	23	Once	Online	tDCS	–	Go/no-go; Stroop; SAT
Soltaninejad et al.47	Anodal	F3/Fp2 (35 cm2 both)	1.5	1	15	Once	Online	tDCS	–	Go/no-go; Stroop
	Cathodal	F3/Fp2 (35 cm2 both)	1.5	1	15	Once	Online	tDCS	–	Go/no-go; Stroop
Soff et al.48	Anodal	F3 (3.14 cm2)/Cz (12.5 cm2)	1	5	20	5 consecutive weekdays	Online	tDCS	FBB-ADHD	QbTest
Nejati et al.49	Anodal	F3/F4 (25 cm2)	1	1	15	Once	Offline	tDCS	–	Go/no-go; Stroop; N-back; WCST
	Anodal	F3/Fp2 (25 cm2)	1	1	15	Once	Offline	tDCS	–	Go/no-go; Stroop; N-back; WCST
	Cathodal	F3/Fp2 (25 cm2)	1	1	15	Once	Offline	tDCS	–	Go/no-go; Stroop; N-back; WCST
Sotnikova et al.37	Anodal	F3 (13 cm2)/Cz (35 cm2)	1	1	20	Once	Online	tDCS	–	Go/no-go; N-back
	Anodal	F8/left mastoid (35 cm2 both)	1	1	20	Once	Online	tDCS	–	Flanker
	Cathodal	F8/left mastoid (35 cm2 both)	1	1	20	Once	Online	tDCS	–	Flanker
Breitling et al.50	Anodal	F8/left mastoid (35 cm2 both)	1	1	20	Once	Online	tDCS	–	Flanker
	Cathodal	F8/left mastoid (35 cm2 both)	1	1	20	Once	Online	tDCS	–	Flanker
Munz et al.51	Anodal	F3 + F4/both mastoids (0.5 cm2 all)	0–0.25 mA (slow oscillations)	1	5 × 5	Once	Offline	Slow-oscillating tDCS	–	Go/no-go; motor memory task; alertness task
Krauel et al.52	Anodal	AF3 + F3/TP7 + Oz (3.14 cm2 both)	1	10	20	2 wk (2 times 5 sessions)	Online	tDCS	ADHD rating scale	N-back
	Anodal	F6 + F8/AFz + P7 (3.14 cm2 both)	1	10	20	2 wk (2 times 5 sessions)	Online	tDCS	–	Flanker
Nejati et al.36	Anodal	F3/Fp2 (25 cm2 both)	2	–	20	–	Online	tDCS	–	Dot probe task; Emotional Stroop
	Anodal	Fp2/F3 (25 cm2 both)	2	–	20	–	Online	tDCS	–	Dot probe task; Emotional Stroop

ADHD = attention-deficit/hyperactivity disorder; ADHD-RS = parent-rated ADHD Rating Scale IV home version; AFz = midline anterior frontal; AF3 = left anterior frontal; AF4 = right anterior frontal; AF8 = right anterior frontal cortex; CANTAB = Cambridge Neuropsychological Test Automated Battery; Cz = midline central; CPRS = Conners’ Parent Rating Scales; CPT = Continuous Performance Task; CT = cognitive training; Fp1 = left frontal pole; Fp2 = right frontal pole; Fpz = midline frontal pole; FBB-ADHD = parents’ version of German adaptive ADHD diagnostic checklist; F3 = left dorsolateral prefrontal cortex); F4 = right middle frontal; F6 = right inferior frontal; F8 = right inferior frontal cortex; IVA + CPT = Integrated Visual and Auditory Continuous Performance Test; Oz = occipital midline; P4 = right posterior parietal cortex; P7 = left posterior temporal; QbTest = Quantitative Behaviour Test; RPM = Raven Progressive Matrices; SNAP-IV = Swanson, Nolan, and Pelham-IV rating scale; SSRT = Stop Signal Reaction Time; tDCS = transcranial direct current stimulation; SAT = Shifting Attention Test; TAVIS-4 = Visual Attention Test, 4th Edition; TMT = trail making test; TP7 = left temporo-parietal junction; VADPRS = Vanderbilt ADHD Parent Rating Scale; WCST = Wisconsin Card Sorting Test.

Risk of bias in included studies

The risk of bias in the included studies is detailed in Figure 2 and Figure 3. Overall, the risk of bias was low. Of the 18 studies, 17 reported randomization procedures, while 2 did not specify allocation concealment methods. Consequently, the selection bias risk for these studies was generally low. One study did not implement blinding for participants, and 7 studies failed to blind assessors. However, given that most outcomes were objectively measured, the risk of detection bias remained low. Notably, 4 studies did not provide accuracy data for experimental conditions either in the main text or appendices. Additionally, 1 study exhibited selective reporting bias. Among the 18 studies, 17 showed no other serious biases; however, 1 study had incomplete baseline data for both the experimental and control groups.

Figure 2.

Overall risk of bias for all included studies. The overall risk of bias was low.

Figure 3.

Risk of bias in included studies. See Related Content tab for accessible version.

Meta-analysis of clinical symptoms of ADHD

Among the 14 studies included in the meta-analysis, 5 specifically examined the effect of tDCS on the clinical symptoms of children with ADHD. The severity of ADHD symptoms was assessed using parent-reported scales, where higher scores indicated milder symptoms after numeric conversion. Specifically, 2 studies used the ADHD Rating Scale-IV, 1 employed the Vanderbilt ADHD Parent Rating Scale, 1 employed the ADHD Rating Scale and another used the FBB-ADHD (Fremdbeurteilungsbogen für hyperkinetische Störungen), a German ADHD rating scale. In a random-effects model, our analysis revealed that tDCS intervention did not significantly improve clinical symptoms among children with ADHD (SMD 0.012, 95% CI −0.235 to 0.259), and with only a slight trend toward improvement (Figure 4).

Figure 4.

The effect of transcranial direct current stimulation (tDCS) on clinical symptoms of attention-deficit/hyperactivity disorder. Weights are from random-effects analysis. CI = confidence interval; SMD = standardized mean difference. See Related Content tab for accessible version. See Appendix 1, Section 5, for group definitions.

Meta-analysis of cognitive function in ADHD

Attention

Twelve of the 14 studies included in the meta-analysis examined the effect of tDCS on the cognitive function, specifically attention, among children with ADHD. The attention performance of these children was objectively assessed with professional equipment, where higher scores indicated better attention ability after numeric conversion. The studies used various neuropsychological tests, whereby 4 used Go/No-Go, 3 used the Continuous Performance Test (CPT), 3 used N-Back tasks, 2 used the Wisconsin Card Sorting Test (WCST), and 1 each used the Stroop Test, the Cambridge Neuropsychological Test Automated Battery (CANTAB), the Rail Making Test, Raven’s Progressive Matrices, the Flanker task, and the fourth edition of the Visual Attention Test (TAVIS-4). In a random-effects model, tDCS intervention significantly improved attention performance among children with ADHD (SMD 0.207, 95% CI 0.011 to 0.403; Figure 5). However, we observed substantial heterogeneity among the studies (I² = 64.5%).

Figure 5.

The effect of transcranial direct current stimulation (tDCS) on attention among children with attention-deficit/hyperactivity disorder. Weights are from random-effects analysis. CI = confidence interval; SMD = standardized mean difference. See Related Content tab for accessible version. See Appendix 1, Section 6, for group definitions.

To explore potential sources of heterogeneity, we conducted subgroup analyses by stimulation site, current intensity, electrode polarity, and number of stimulation sessions. Compared with other sites (e.g., F8, F3 + F4, AF3 + F3, P4), tDCS of the F3 site significantly enhanced attention (Figure 6). Current intensities of 1 mA, 1.5 mA, and 2 mA did not significantly affect attention; however, a current intensity of 1 mA showed a more trend toward improving attention more than 1.5 mA and 2 mA (Appendix 1, Section 2.1). Cathodal stimulation was more effective than anodal stimulation (Appendix 1, Section 2.2). Relative to 5, 10, or 15 sessions, single sessions or 12 sessions of stimulation led to significant improvements (Appendix 1, Section 2.3).

Figure 6.

Subgroup analysis of effect of transcranial direct current stimulation (tDCS) by stimulation site. Weights are from random-effects analysis. AF3 = left anterior frontal; CI = confidence interval; F3 = left dorsolateral prefrontal cortex; F4 = right middle frontal; F8 = right inferior frontal cortex; P4 = right posterior parietal cortex; SMD = standardized mean difference. See Related Content tab for accessible version. See Appendix 1, Section 6, for group definitions.

Inhibitory control

Eleven of the 14 studies included in the meta-analysis examined the effect of tDCS on inhibitory control among children with ADHD. The inhibitory control ability of these children was objectively assessed with professional equipment, where higher scores after numeric conversion indicated better inhibitory control. Specifically, 6 studies used Go/No-Go tasks, 3 used the CPT, 2 used the WCST, 3 used Stroop tests, 1 used CANTAB, 3 used Flanker tasks, and 1 used TAVIS-4. In a random-effects model, tDCS intervention significantly improved inhibitory control among children with ADHD (SMD 0.222, 95% CI 0.045 to 0.399; Figure 7). However, substantial heterogeneity was observed (I² = 55.6%).

Figure 7.

The effect of transcranial direct current stimulation (tDCS) on inhibitory control. Weights are from random-effects analysis. CI = confidence interval; SMD = standardized mean difference. See Related Content tab for accessible version. See Appendix 1, Section 7, for group definitions.

To explore potential sources of heterogeneity, we conducted subgroup analyses by stimulation site, current intensity, electrode polarity, and number of stimulation sessions. Compared with other sites (e.g., F8, F6 + F8, F3 + F4), F3 stimulation significantly enhanced inhibitory control (Figure 8). A current intensity of 1 mA showed greater improvement than 1.5 mA and 2 mA (Appendix 1, Section 3.1). Cathodal stimulation was more effective than anodal stimulation (Appendix 1, Section 3.2). A single session of stimulation led to significant improvements compared with 5, 10, 12, or 15 sessions (Appendix 1, Section 3.3).

Figure 8.

Subgroup analysis of effect of transcranial direct current stimulation (tDCS) by stimulation site. Weights are from random-effects analysis. CI = confidence interval; F3 = left dorsolateral prefrontal cortex; F4 = right middle frontal; F6 = right inferior frontal; F8 = right inferior frontal cortex; SMD = standardized mean difference. See Related Content tab for accessible version. See Appendix 1, Section 7, for group definitions.

Processing speed

Eight of the 14 studies included in the meta-analysis examined the effect of tDCS on processing speed among children with ADHD. The processing speed of these children was objectively assessed using professional equipment, where higher scores after numerical conversion indicated faster processing speed. Specifically, 5 studies used Go/No-Go tasks, 2 used Simon Tasks, 2 used the WCST, 2 used N-Back tasks, 1 used an alertness task, and 1 used a speed of processing test. In the random-effects model, tDCS intervention did not significantly improve processing speed (SMD 0.063, 95% CI −0.145 to 0.270; Figure 9). However, we observed moderate heterogeneity among the studies (I² = 38.6%).

Figure 9.

The effect of transcranial direct current stimulation (tDCS) on processing speed. Weights are from random-effects analysis. CI = confidence interval; SMD = standardized mean difference. See Related Content tab for accessible version. See Appendix 1, Section 8, for group definitions.

To explore potential sources of heterogeneity, we conducted subgroup analyses by stimulation site, current intensity, electrode polarity, and number of stimulation sessions. Compared with other sites (e.g., F8, F3 + F4, AF3 + F3, P4) F3 stimulation significantly improved processing speed (Figure 10). Current intensities of 1 mA, 1.5 mA, and 2 mA did not significantly affect processing speed (Appendix 1, Section 4.1). Neither cathodal nor anodal stimulation showed significant improvements (Appendix 1, Section 4.2). The number of stimulation sessions (1 session, 10 sessions, or 15 sessions) had no significant effect on processing speed (Appendix 1, Section 4.3).

Figure 10.

Subgroup analysis of stimulation sites. Weights are from random-effects analysis. AF3 = left anterior frontal; CI = confidence interval; F3 = left dorsolateral prefrontal cortex; F4 = right middle frontal; F8 = right inferior frontal cortex; P4 = right posterior parietal cortex; SMD = standardized mean difference; tDCS = transcranial direct current stimulation. See Related Content tab for accessible version. See Appendix 1, Section 8, for group definitions.

Systematic review

Three double-blind, randomized, sham-controlled studies involving 98 children with ADHD examined the effects of tDCS combined with cognitive training.39,41,42 Clinical symptom questionnaires were used to assess outcomes. Two studies39,41 applied 15 sessions of anodal tDCS to the right inferior frontal cortex (F8), while 1 study42 applied 12 sessions of anodal tDCS to the left dlPFC (F3). Compared with sham tDCS, neither the F8 or F3 anodal tDCS showed significant improvements in ADHD symptoms or cognitive performance. Specifically, no significant differences were observed in any of the primary outcome measures.

Two randomized sham-controlled studies involving 116 children with ADHD examined the effects of 10 sessions of anodal tDCS (1 mA current intensity) applied to the Fp2, AF3 + F3, or F8 + F6 sites.34,52 Both studies used clinical symptom questionnaires for assessment. Results from 1 study34 indicated that, compared with sham tDCS, anodal tDCS significantly reduced commission errors on the Integrated Visual and Auditory CPT, suggesting a positive effect on attention maintenance and inhibitory control. However, there was no significant improvement in the interference reaction time of colour and word in the Stroop task. No significant differences were observed in the total number of steps completed in the Tower of Hanoi task between the 2 groups, but the total completion time gradually decreased over the course of the intervention in both groups. Notably, compared with the sham tDCS group, the tDCS group showed more significant reductions in completion time on the Tower of Hanoi task after the 5th session, after the 10th session, and at the 6-week follow-up. Although tDCS stimulation of the right orbitofrontal cortex did not significantly improve overall ADHD symptoms, it significantly enhanced cognitive indicators related to attention maintenance. Another study found that, compared with sham tDCS, anodal tDCS of the left DLPFC led to poorer accuracy in working memory, whereas anodal tDCS of the right inferior frontal gyrus significantly improved interference control and increased accuracy in working memory and continuous performance tasks, supporting the role of the of the right inferior frontal gyrus as an important network hub.52 Clinical effects were less clear. Ratings of ADHD symptoms appeared to decline after the treatment and at follow-up, but this effect was not significantly stronger in the anodal tDCS group. These findings could suggest different trajectories for (direct) neuropsychological and (indirect) clinical effects.

A single-blind, randomized, sham-controlled study involving 26 children with ADHD applied a single session of anodal tDCS (1 mA current intensity) to the F3 site.35 Cognitive features were assessed using the Stop Signal Reaction Time and N-back task. A single session of active tDCS to the F3 site did not demonstrate significant improvements relative to baseline or sham tDCS. Specifically, there was no consistent enhancement in neurocognitive performance across the evaluated measures.

A double-blind, randomized, sham-controlled crossover study involving 13 children with ADHD applied a single session of anodal tDCS (1 mA current intensity) to the F3 site.37 Compared with sham stimulation, the tDCS group exhibited shorter reaction times in the 2-back task. Among adolescents with ADHD, tDCS had a significant effect on working memory performance, particularly for tasks with high complexity and high memory load. Specifically, tDCS appeared to mitigate the increase in mental effort associated with higher memory loads and more complex tasks.

A randomized, single-blinded, sham-controlled, crossover study involving 22 children with ADHD applied anodal tDCS (2 mA current intensity) to the F3 and Fp2 sites, separately, to evaluate the effect of stimulation on attention bias to positive and negative faces.36 The results indicated a reduction in attention bias during anodal tDCS of the left DLPFC prefrontal cortex, coupled with cathodal tDCS to the right ventromedial prefrontal cortex, and during anodal tDCS to the right ventromedial prefrontal cortex and cathodal tDCS of the left DLPFC, compared with sham stimulation. This result was limited to the emotional Stroop test, with a longer presentation time and greater executive demand than the dot probe task. The effect of tDCS on attention bias was independent of valence.

Across studies, the overall tolerability of tDCS was favourable, with the most frequently reported adverse effects being mild tingling and itching (Table 3). One study reported that 3 participants in the tDCS group had notable headaches, which resulted in the discontinuation or temporary suspension of their involvement in the study.42

Table 3.

Adverse effects and reactions reported in transcranial direct current stimulation (tDCS) studies

Study	Adverse effects and reactions
Westwood et al.30	There were no significant group differences in ratings of mood, wakefulness, overall impression of tDCS and cognitive training, and adverse effects. Adverse effects were significantly higher after anodal versus sham tDCS at posttreatment. Stimulation was well tolerated, with significantly higher reports of burning sensation during anodal only.
D’Aiello et al.35	Most participants (92%) did not experience any adverse effects other than itching during both active and sham stimulation.
Wang et al.34	Participants reported mild and transient unfavourable effects such as tingling or itching of the scalp.
Guimarães et al.40	Adverse events were mostly self-limiting and characterized as mild to moderate. Pruritus was identified in 60% of children from the tDCS group and 20% from the sham group. Tingling and burning of greater intensity were reported by 26.7% and 20% children, respectively, from the tDCS and sham groups.
Westwood et al.41	There were no significant differences between anode tDCS and sham groups based on adverse reactions and effect outcome indicators.
Schertz et al.42	The overall adverse effects reported in this study were mild and resolved after the intervention, with no significant differences found between the tDCS and sham groups. However, 3 participants from the tDCS group developed headaches, leading to cancellation or temporary suspension of the study.
Salehinejad et al.43	The stimulation was well tolerated with a few participants reporting mild adverse effects during the stimulation, There was no significant difference in headache, dizziness, burning sensation and itching between anode tDCS and sham groups.
Makkar et al.44	Not reported.
Klomjai et al.45	Not reported.
Salehinejad et al.46	The stimulation was well tolerated. Only mild adverse effects were reported during the stimulation. A trendwise enhanced itching sensation was reported during active stimulation.
Soltaninejad et al.47	Not reported.
Soff et al.48	Forty-six percent of participants reported tingling and itching sensations under electrode during sham and anodal tDCS. No participants reported fatigue, burning, pain, or other uncomfortable sensations during tDCS or sham stimulation.
Nejati et al.49	All participants tolerated tDCS well and no adverse effects were reported except for mild itching or tingling under electrodes.
Sotnikova et al.37	A mild tingling and itching sensation under the electrodes was reported by 46% of participants during both anodal tDCS and sham stimulation.
Breitling et al.50	Skin sensations were rated higher during cathodal tDCS than sham tDCS.
Munz et al.51	Not tested.
Krauel et al.52	Serious adverse events did not occur. All adverse effects were transient and resolved by the end of the trial. The most frequent adverse effect in both groups was headache, followed by nasopharyngitis and feeling of electric discharge.
Nejati et al.36	All participants tolerated the stimulation without major adverse effects.

Discussion

In recent years, the number of studies involving tDCS for ADHD has increased; however, considerable heterogeneity exists in stimulation protocols and measurement outcomes. To address this issue, we conducted a rigorous meta-analysis and systematically reviewed studies from which data could not be extracted. In our analysis, we separately evaluated clinical symptoms and cognitive functions, with cognitive functions further categorized into attention, inhibitory control, and processing speed to enhance homogeneity. The results indicated that tDCS did not significantly improve clinical symptoms or processing speed among children with ADHD, although there was a slight trend toward improvement. Attention and inhibitory control improved significantly with tDCS, but given the relatively small effect size, we can only suggest that tDCS may have some positive effect on the specific cognitive functions of children with ADHD. These preliminary results are partially consistent with previous meta-analyses.30,53

To explore sources of heterogeneity and optimize tDCS protocols, our meta-analysis specifically compared the efficacy of different stimulation sites, current intensities, electrode polarities, and stimulation durations, providing deeper insights into the effects of tDCS on children with ADHD. Only F3 stimulation significantly enhanced attention, inhibitory control, and processing speed compared with other regions (e.g., F8, P4, F3 + AF3, F8 + F6, F3 + F4). Regarding current intensity, 1 mA significantly improved inhibitory control compared with 1.5 mA and 2 mA. Cathodal stimulation was more effective than anodal stimulation in improving attention and inhibitory control. In terms of the number of sessions, a single tDCS session yielded significant improvements in attention and inhibitory control compared with multiple sessions (5, 10, 12, or 15 sessions). However, these findings require validation through rigorous and large-scale studies.

Abnormalities in the striatofrontal pathway and the prefrontal cortex are physiologically associated with the cognitive–behavioural problems of ADHD.54,55 The effect of tDCS on attention may be related to the fact that meta-analyses of functional MRI studies have shown that attention is mainly mediated by the DLPFC56,57 and IFC,58,59 while inhibitory function is mainly mediated by the right IFC.58,60,61 Patients with ADHD often show low metabolism and low activity in the DLPFC and IFC,62 and a meta-analysis has shown that the right IFC is under-activated during inhibitory control.63 Transcranial direct current stimulation is a potential means of regulating cortical excitability and inducing plasticity in the human brain.29,64 Given the polarity-specific effect, anodal tDCS can increase cortical excitability, while cathodal tDCS can reduce cortical excitability within a certain dose.65 Therefore, tDCS may improve the attention function of ADHD by stimulating the dlPFC and IFC, and stimulating the dlPFC can improve the attention and inhibitory control among children with ADHD, which is consistent with the results of this study. However, our subgroup analysis found that cathodal stimulation was more effective than anodal stimulation. This may be owing to the relatively small number of studies on cathodal tDCS. Single cathodal stimulation of the left DLPFC can improve inhibitory control and selective attention in ADHD by inhibiting the right hemisphere through the corpus callosum.49 This hypothesis is supported by studies involving healthy adults, which have shown that correct IFC stimulation can improve inhibitory performance.66 The subgroup analysis of this study also found that single stimulation was more effective than multiple stimulations. We speculate that this may be owing to the immediate effect of tDCS on cognitive function, with assessments after single stimulation showing more significant improvement than multiple sessions. The subgroup analysis of current intensity found that 1 mA was better than 1.5 mA and 2 mA. Most studies chose 1 mA for the stimulation protocol, while studies choosing 1.5 mA and 2 mA were relatively few, which may have contributed to this finding. Moreover, for children and adolescents, small current stimulation may be more acceptable.

The trend of improvement in processing speed by tDCS stimulation of the left DLPFC is consistent with adult fMRI evidence, indicating that the left DLPFC is a key brain region for processing speed.67,68 This study found that stimulation of the right IFC led to a completely opposite result in processing speed, with the sham control group showing significantly better processing speed results than the tDCS group. However, the results of studies that stimulated only the left DLPFC showed that tDCS significantly improved processing speed, which is consistent with neuroimaging evidence that the left DLPFC mediates processing speed and that the left DLPFC is the best site for improving processing speed in ADHD. These findings are partially consistent with the meta-analysis by Salehinejad and colleagues, 53 which indicated that tDCS on the left DLPFC mainly improved processing speed in patients with ADHD.

Regarding the safety profile of tDCS, the stimulation was generally well tolerated. One study reported that 3 participants in the tDCs group experienced notable headaches, which led to the discontinuation or temporary suspension of their involvement.42 In most studies, the intervention group reported mild sensations such as tingling, itching, and burning.

Overall, tDCS has the potential to improve the attention and inhibitory abilities of children with ADHD. However, the diversity of stimulation protocols presents a serious challenge in identifying the most effective treatment regimen. To better understand the effect and mechanisms of tDCS on clinical symptoms and cognitive functions in children with ADHD, future research should prioritize developing tailored stimulation protocols for different ADHD subtypes to enhance treatment specificity. Currently, the sample sizes in existing studies are relatively small, and future research should aim to increase sample sizes to validate these findings.

Limitations

Subgroup analysis of highly heterogeneous results revealed that different tDCS protocols influenced treatment outcomes. However, the limited number of studies in some subgroups may have compromised the reliability of these findings, highlighting the need for additional relevant clinical trials. Some studies did not clearly report the random sequence generation or allocation concealment. This study did not provide a detailed delineation of symptomatic subtypes (e.g., inattentive, hyperactive–impulsive) among children with ADHD, which may have affected the specific analysis of the efficacy of tDCS. In this study, we conducted a single-dimensional subgroup analysis, with no combined-dimensional subgroup analysis, which allowed only a single analysis of which stimulation site, current intensity, electrode polarity, or number of stimulation sessions had an improvement effect on ADHD. We could not analyze the effects of complete stimulation plans.

Conclusion

The evidence from this meta-analysis showed that tDCS can improve attention and inhibitory control among children with ADHD. Specifically, for attention and inhibitory control, stimulation of the left DLPFC is more effective than stimulation of the right IFC, bilateral DLPFC, and the right posterior parietal cortex. A current intensity of 1 mA is superior to 1.5 mA and 2 mA, and cathodal stimulation is more effective than anodal stimulation. For processing speed, stimulation of the left dlPFC was found to be more effective than stimulation of the right IFC, bilateral DLPFC, and right posterior parietal cortex. Given the limited number of included studies, future research should involve more rigorous RCTs and meticulously designed stimulation protocols.