Tiffany Carther-Krone1,2
Ji Hyun Ko1,2,3*- 1Department of Human Anatomy and Cell Science, Max Rady College of Medicine, University of Manitoba, Winnipeg, MB, Canada
- 2PrairieNeuro Research Centre, Kleysen Institute for Advanced Medicine, Winnipeg Health Science Centre, Winnipeg, MB, Canada
- 3Graduate Program in Biomedical Engineering, Price Faculty of Engineering, University of Manitoba, Winnipeg, MB, Canada
Background: High-definition transcranial direct current stimulation (HD-tDCS) is a non-invasive brain stimulation technique that offers increased spatial precision compared to conventional tDCS. As its use has expanded across research and clinical settings, there has been increasing interest in understanding its safety and tolerability.
Objective: This review summarizes adverse events related to HD-tDCS in both healthy and clinical populations, focusing on how stimulation intensity, session frequency, and polarity influence tolerability.
Results: In healthy populations, HD-tDCS is most often administered at 1–2 mA for 20 min. The most reported adverse events include tingling, itching and burning localized to the site of stimulation, typically described as mild or transient. Studies comparing active and sham stimulation generally report no significant differences in adverse event frequency or intensity, even at higher intensities of 2–3 mA. Reports of severe adverse events are rare, and participant dropout due to discomfort is uncommon. Multi-session protocols show similar safety profiles, suggesting that repeated stimulation does not increase adverse effects. In clinical populations HD-tDCS is typically delivered across multiple sessions. Reported adverse events are mild and transient, with few reports of severe outcomes. Polarity-specific comparisons suggest that anodal and cathodal stimulation are similarly tolerated, with no notable differences in adverse event profiles.
Conclusion: Overall, current evidence indicates that HD-tDCS is a safe and well-tolerated technique across diverse populations and stimulation parameters. Continued use of standardized adverse event reporting will be important to further confirm these findings as clinical application broaden.
Highlights
• Healthy and clinical populations were not found to be particularly vulnerable to HD-tDCS.
• The most commonly reported adverse effects were itching, tingling and burning sensation, typically described as mild or transient.
• Common adverse effects were localized to the site of stimulation.
• Multi-session protocols show similar safety profiles to single-session protocols.
Background
Over the last 20 years non-invasive brain stimulation (NIBS) techniques, such as transcranial direct current stimulation (tDCS), have received increased interest (Ruffini et al., 2013). In conventional tDCS two rubber electrodes are placed in saline-soaked sponges over the scalp such that the ingoing current projects from an electrode placed over the target brain region to a return electrode placed over a different brain region or an extracranial area (Figure 1a). This setup delivers a relatively weak current through the cortex, aiming to modulate brain function (Pascual-Leone and Fregni, 2007; Nitsche et al., 2007). However, the size of the tDCS electrodes/sponges has resulted in relatively non-focal stimulation (Lang et al., 2005), and often affects not only the targeted region but also intervening areas between the two electrodes, as demonstrated by computational modeling studies (Datta et al., 2009; Faria et al., 2011).
Figure 1. Comparisons of electrode configurations in transcranial direct current stimulation (tDCS). (a) In conventional tDCS, two large rubber electrodes are placed in saline-soaked sponges over the scalp. Current flows from the anodal electrode placed over the target brain region to a return electrode positioned over another brain region or an extracranial area. (b) High-definition tDCS (HD-tDCS) increases focality by using smaller electrodes arranged in specific configurations. One commonly used setup is the 4×1 ring montage, in which a central electrode is surrounded by four return electrodes arranged in a circular pattern to constrain current flow and improve focality. (c) More complex HD-tDCS configurations use montages with more than five electrodes, involving multiple anodal and cathodal sites. Here the right dorsolateral prefrontal cortex is targeted. The cutout shows the simulated electrical current in a head model based on magnetic resonance imaging using HDTargets (Soterix, Inc).
High-definition tDCS (HD-tDCS) is a relatively recent modification of conventional tDCS that increases focality by using multiple smaller electrodes (Kuo et al., 2013; Villamar et al., 2013a). While electrode configurations are often guided by computational modeling approaches to optimize current delivery to the target region (Edwards et al., 2013; Dmochowski et al., 2011), one of the most commonly used montage set-ups is the “4×1 ring montage,” in which a central electrode is surrounded by four return electrodes arranged in a circular formation (Figure 1b). The configuration serves to constrain current flow to the region between the center and ring electrodes (Villamar et al., 2013a) and improve the focality of stimulation compared to the traditional bipolar montage of conventional tDCS (Datta et al., 2009; Edwards et al., 2013). Another commonly used configuration involves montages with more than five electrodes, in which multiple anodal and cathodal sites are defined to steer and confine current flow toward targeted brain regions (Figure 1c). These more complex arrangements further enhance focality and allow for greater precision in guiding the current flow.
This increased focality of HD-tDCS, along with its non-invasiveness and its ability to induce changes in cortical excitability that may outlast those of conventional tDCS (Kuo et al., 2013) has contributed to its growing popularity in both cognitive neuroscience and clinical research. HD-tDCS has been most commonly applied to modulate a variety of cognitive functions including attention (Lu et al., 2020; Nikolin et al., 2019; Martínez-Pérez et al., 2020), memory (Nikolin et al., 2019; Nikolin et al., 2015; Chua et al., 2017), and executive functions (Imburgio and Orr, 2018; Lu et al., 2021). Clinically, it has been investigated in neuropsychiatric and neurological conditions such as fibromyalgia (Villamar et al., 2013b; Castillo-Saavedra et al., 2016), mild cognitive impairment (Rezakhani et al., 2024; Iordan et al., 2022), post-traumatic stress disorder (Hampstead et al., 2020), depression (Salehinejad et al., 2023; Wong et al., 2019), and tinnitus (Jacquemin et al., 2021). The use of the 4×1 ring montage is particularly common in these contexts because it provides improved focality over conventional bipolar tDCS, allowing researchers to target relatively constrained cortical regions such as the dorsolateral prefrontal cortex. More complex multi-electrode montages have been motivated by the need to direct current toward distributed but functionally connected cortical regions, thus expanding the potential applications of HD-tDCS. Direct comparisons of HD-tDCS and conventional tDCS remain relatively limited but generally support comparable safety and tolerability. For example, Jacquemin et al. reported that fewer sensations were experienced during HD-tDCS than conventional tDCS in 39 patients with tinnitus (Jacquemin et al., 2018), while da Silva Machado et al. found that both techniques were similarly well tolerated in 12 high endurance athletes (da Silva Machado et al., 2021). To date, a few studies have demonstrated superior or faster benefits of HD-tDCS over conventional tDCS in clinical populations (Zeng et al., 2024; Jog et al., 2025), but more studies need to be done for a proper meta-analysis.
Despite the increasing use of HD-tDCS, its safety profile remains less established than that of conventional tDCS. While the majority of reported adverse effects in conventional tDCS are mild and transient, such as tingling or itching under the electrodes, some studies have noted more persistent effects—most commonly skin irritation—that may last beyond the stimulation period. Given differences in electrode configurations, current density distributions, and overall stimulation profiles, safety findings from conventional tDCS cannot be assumed to directly translate to HD-tDCS.
The aim of this review is to summarize the current literature on adverse events associated with HD-tDCS in both healthy and clinical populations, focusing on the impact of stimulation parameters, number of HD-tDCS sessions and polarity. By consolidating available evidence, this review seeks to provide researchers and clinicians with clearer insight into the tolerability and risks associated with HD-tDCS, and to support the ongoing development of evidence-based safety guidelines for its use.
Adverse effects of HD-tDCS
Since the introduction of HD-tDCS in 2009, safety and tolerability have been at the forefront of study design protocols. Early work focused on optimizing “high-definition” electrode-gel parameters for electrode durability, skin safety, and subjective pain, testing various parameters such as anode and cathode electrode potential, temperature, pH, and subjective sensation during the application of 2 mA direct current, for up to 22 min, on each subjects’ forearms (Minhas et al., 2010). Using this protocol, no serious adverse events such as skin burning were observed. Minor adverse effects included transient redness at the site of the electrodes, skin irritation in the form of small bumps of black dots (<1 mm) and apparent roughening of the skin under the electrodes. All effects on the skin were reversible and disappeared within a few hours, and no subject reported lasting irritation or pain.
Following computational modeling studies suggested that replacing the two large sponge electrodes used in conventional tDCS with an array of smaller electrodes could improve targeting (Dmochowski et al., 2011). Subsequent research using a 4 × 1 ring configuration demonstrated that this targeted montage could successfully induce changes in neuronal excitability while maintaining a favorable safety profile (Caparelli-Daquer et al., 2012). Participants reported only mild sensations like itching or tingling, and none requested to terminate stimulation. Building on these early demonstrations of safety and tolerability, a growing number of studies have since applied HD-tDCS across diverse healthy and clinical populations, consistently reporting minimal and transient side effects.
Healthy populations
HD-tDCS is generally well tolerated in healthy adults, with adverse events that are mild, transient, and comparable to conventional tDCS. The most frequently reported sensations include tingling, itching, or a mild burning sensation at the site of the electrode (Table 1). These sensations typically occur in both active and sham conditions and are well tolerated. Across studies, serious adverse events are rare, and discomfort typically does not interfere with task performance or study completion. Nonetheless, differences in stimulation parameters, session number, and polarity may influence the occurrence of side effects, and a thorough examination of these parameters is critical for guiding safe research and clinical applications.
Table 1. Study design and adverse event reports for healthy populations.
Adverse events by stimulation parameters
The intensity and duration of stimulation are two primary parameters that influence the incidence and nature of adverse events in HD-tDCS studies. Across the reviewed literature, stimulation currents ranged from 1 mA to 3 mA, with durations spanning 10 to 30 min. A consistent trend emerged in which higher intensities and longer durations were associated with a modest increase in the frequency of common adverse events, primarily tingling, itching, warmth, and mild headache. Yet even at higher intensities and longer durations adverse events were mainly mild in severity and transient.
Across studies administering HD-tDCS at intensities between 1 and 1.5 mA for durations of 10 to 20 min, the most common sensations were mild and transient including burning, tingling, and itching under the site of the electrodes (Caparelli-Daquer et al., 2012; Roy et al., 2014; Turski et al., 2017; Lenoir et al., 2017; Ke et al., 2019; Fresnoza et al., 2024; Jin et al., 2025). Occasional reports of headache were also noted, typically resolving without intervention (Turski et al., 2017). In most cases, adverse event ratings were described as mild (Huang et al., 2021; Schmidt et al., 2024; Schroeder et al., 2024), and comparisons between active and sham stimulation groups showed no significant differences across symptoms such as headache, neck/scalp pain, tingling, itching, burning, sleepiness, concentration, mood changes, and skin irritation (Gbadeyan et al., 2016b; Gbadeyan et al., 2016a; Pixa et al., 2017a; Martin et al., 2017a; Martin et al., 2017b; Perceval et al., 2017; Gbadeyan et al., 2019; Behroozmand et al., 2020; Splittgerber et al., 2020; Martin et al., 2021; Berglund-Barraza et al., 2023; Otstavnov et al., 2024; Hill et al., 2018; Naka et al., 2018). This finding was similarly observed at 1.5 mA (Hill et al., 2018; Naka et al., 2018; Hill et al., 2019; Guo et al., 2018). When participants did withdraw from a study, the primary reason cited was discomfort during stimulation (Hill et al., 2018).
In studies that did report differences between stimulation and sham groups at 1 mA, one study found that effects were limited to the first 5 min of stimulation, with ratings comparable to that of the sham group by the end of the stimulation session (Hill et al., 2017). Other studies found slightly higher ratings for specific sensations, such as scalp pain (Martin et al., 2017a; Martin et al., 2017b) or itching/tingling (Schroeder et al., 2024), in the active group, but overall adverse event profiles remained mild and comparable between conditions. Similar patterns were observed at 1.5 mA, with higher ratings of pain/discomfort (Sedgmond et al., 2020) and skin tingling (Lu et al., 2021) in the active compared to sham group, but no severe or lasting effects.
In some cases the sham group has been found to report higher sensation ratings compared to the active group. For example, one study found that participants in the sham group reported increased ratings of the experience of tingling compared to that of the active group, although ratings of a burning sensation were higher for the active group compared to the sham group (To et al., 2018). In other cases, no evidence for the effect of stimulation on adverse events has been found at all for 1 mA (Martin et al., 2019b; Martin et al., 2019a; Stefano et al., 2022), 1.5 mA (Wen et al., 2019; Dai et al., 2021), 2 mA (Stefano et al., 2022; Flood et al., 2016; Shen et al., 2016; Kumar et al., 2020; Xiao et al., 2020; Wan et al., 2021; Kold & Graven-Nielsen, 2022; Kold & Graven-Nielsen, 2021; Shen et al., 2024; Wang L. et al., 2024) or 3 mA (Stefano et al., 2022).
At 2 mA, HD-tDCS was again associated with sensations such as tingling, burning, and itching (Kuo et al., 2013; Nikolin et al., 2019; Borckardt et al., 2012; Zhou et al., 2019; Xiong et al., 2023; Tabari et al., 2024; Kipping et al., 2024). While most reports were cited as being “weak” (Panico et al., 2022) or “mild” (Choi and Perrachione, 2019; Gan et al., 2019; Ballard et al., 2021; Jiang et al., 2022; Johari and Berger, 2023; Cacciamani et al., 2024), in some cases they were reported as “moderate” (Chua et al., 2017; Choi and Perrachione, 2019; Gan et al., 2019; Weintraub-Brevda and Chua, 2019) or “severe” (Chua et al., 2017; Gan et al., 2019; Weintraub-Brevda and Chua, 2019; Donaldson et al., 2019). Across most trials, comparisons between active and sham conditions revealed no significant differences in reported symptoms (da Silva Machado et al., 2021; Behroozmand et al., 2020; Tabari et al., 2024; Choi and Perrachione, 2019; Johari and Berger, 2023; Weintraub-Brevda and Chua, 2019; Garnett and den Ouden, 2015; Price et al., 2016; Besson et al., 2019; Lang et al., 2019; Hartmann et al., 2020), and ratings of discomfort, when present, tended to be low. In one study in which participants were administered stimulation at 0.5 mA and 2 mA, perceived sensation ratings were found to be higher in the 2 mA vs. 0.5 mA condition at 30 s after stimulation start, but converged over time (Brunyé et al., 2014). Some studies observed slightly elevated ratings of tingling/itchiness (Xiao et al., 2023), pain (Nikolin et al., 2019; Savic et al., 2019) or sleepiness (Reckow et al., 2018) ratings in the active condition, while others observed that the frequency (Kipping et al., 2024) or strength (Kipping et al., 2024; Lerner et al., 2021) of adverse effects were significantly higher for the stimulation groups compared to the sham group. While a couple of self-resolving headaches were reported (Nikolin et al., 2015), participants generally tolerated stimulation at 2 mA well. When participants withdrew at this intensity, it was usually due to sensations described as “needle-prickling” (Nikolin et al., 2015), unpleasant (Chua et al., 2017), stinging/burning (Nikolin et al., 2019), or general discomfort (Ballard et al., 2021).
While severe adverse events were infrequently reported, they were not entirely absent. Severe sensations, such as tingling, burning, headache, sleepiness and redness, were observed in a small number of participants and were typically associated with 2 mA stimulation using a 4 × 1 ring montage for 20 min at frontal, temporal or parietal sites (Chua et al., 2017; Gan et al., 2019; Weintraub-Brevda and Chua, 2019; Donaldson et al., 2019). For example, one study reported multiple instances of severe burning, headache, tingling, and redness across 39 participants, with one participant withdrawing prior to stimulation due to discomfort (Chua et al., 2017). Another study noted six reports of severe adverse events among 60 participants, including two in the sham group (Weintraub-Brevda and Chua, 2019). While severe adverse events were reported in these two studies, it is unclear how many of these reports came from the same participant. In studies where reporting is clearer, severe side effects were limited to one or two participants. For example, one study reported that one out of 22 participants experienced severe tingling (Gan et al., 2019) while another reported two of 53 participants temporarily ceased stimulation due to discomfort but resumed after a short 1–3 min break (Donaldson et al., 2019). In most cases, even participants experiencing stronger sensations completed the study, and reported symptoms resolved quickly.
Fewer studies have explored stimulation above 2 mA, but preliminary findings indicate that tolerability remains high. For example, in studies administering 3 or 4 mA over 20–30 min, no serious adverse events were observed (Reckow et al., 2018). In one large-scale study involving over 3,000 HD-tDCS sessions in older adults, the vast majority of sensations were rated as “none” or “mild,” regardless of whether the stimulation was active or sham (El Jamal et al., 2023). Interestingly, some mild sensations, such as tingling or itching, were reported more frequently at lower intensities (< 2 mA), while skin redness was slightly more common at higher intensities (> 3 mA). These findings suggest that HD-tDCS is well tolerated even at intensities up to 4 mA, although further research is needed to confirm these outcomes across broader populations.
Adverse events by number of HD-tDCS sessions
Most HD-tDCS studies in healthy individuals have employed single-session protocols and consistently report mild, short-lived sensations such as tingling, itching, burning and prickling (Lenoir et al., 2017; Huang et al., 2021; To et al., 2018; Wen et al., 2019),—if any—without adverse effects on participant well-being or task engagement. In many of these studies, no significant differences in adverse event ratings are found between active and sham stimulation groups (Schmidt et al., 2024; Behroozmand et al., 2020; Berglund-Barraza et al., 2023; Otstavnov et al., 2024; Naka et al., 2018; Guo et al., 2018; Choi and Perrachione, 2019; Weintraub-Brevda and Chua, 2019; Lang et al., 2019; Savic et al., 2019; Pixa et al., 2017b). When differences do arise, they typically involve slightly higher ratings of pain (Nikolin et al., 2019) or discomfort (Lerner et al., 2021) in the active group, while overall sensation ratings remain mild. For example, one study reported increased itching and tingling during active stimulation but found no differences in ratings of other sensations such as pain, burning, warmth/heat, iron taste, exhaustion and headache (Schroeder et al., 2024).
Multi-session protocols in which participants experience both active and sham stimulation provide further insight into the tolerability of HD-tDCS. These studies frequently report no significant differences in the incidence (da Silva Machado et al., 2021; Splittgerber et al., 2020; Martin et al., 2021) or intensity (Gbadeyan et al., 2016b; Gbadeyan et al., 2016a; Gbadeyan et al., 2019; Tabari et al., 2024; Garnett and den Ouden, 2015; Price et al., 2016; Hartmann et al., 2020) of adverse event ratings across conditions, including sensations such as pain, tingling, itching, burning, sleepiness, concentration, mood changes and skin irritation. Other studies have found no difference in cutaneous sensations in general (Hill et al., 2018; Hill et al., 2019; Besson et al., 2019). In cases where adverse events are assessed over the course of a stimulation session, one study observed an initial difference in ratings of itchiness between groups that was found to resolve over the course of the 15-min stimulation session (Hill et al., 2017). Another study measured ratings of pain and unpleasantness at the beginning, halfway through, or at the end of the stimulation and found that ratings did not differ between groups across time (Johari and Berger, 2023). While some studies do show higher ratings in the active group compared to the sham group for adverse events such as scalp pain (Martin et al., 2017a; Martin et al., 2017b), pain/discomfort (Sedgmond et al., 2020), unpleasant tingling/itchiness (Kipping et al., 2024; Xiao et al., 2023), and for burning (Imperio and Chua, 2024), a number of studies also report no adverse events at all (Martin et al., 2019b; Martin et al., 2019a; Stefano et al., 2022; Dai et al., 2021; Flood et al., 2016; Shen et al., 2016; Xiao et al., 2020; Wan et al., 2021; Shen et al., 2024; Wang L. et al., 2024), underscoring the favorable safety profile of HD-tDCS.
Repeated-session protocols, where participants receive the same stimulation condition over several sessions, also support the tolerability of HD-tDCS. Across studies ranging from 3 to 20 sessions and intensities between 1 to 2 mA, reported adverse events are typically mild, including sensations such as transient tingling, skin redness, slight discomfort (Lu et al., 2021; Ke et al., 2019), or none at all (Kold and Graven-Nielsen, 2021; Kold and Graven-Nielsen, 2022). In one study involving 20 sessions, no participants withdrew, and no lasting effects were observed (Turski et al., 2017). Another large study involving varied session counts and stimulation intensities in older adults reported no serious adverse events across more than 100 participants (Reckow et al., 2018). While isolated reports of severe sensations (e.g., burning or headache) do exist (Chua et al., 2017), including some in sham groups, such instances are uncommon and typically do not result in withdrawal from the study. Overall, these findings suggest that HD-tDCS is well tolerated in healthy individuals, even with repeated exposure, although continued large-scale studies with systematic tracking of adverse effects are warranted.
Adverse events by polarity: anodal vs. cathodal
A few studies using either a 3×1 or 4×1 ring montage have employed both anodal and cathodal HD-tDCS to investigate polarity-specific effects. Across these studies, the most commonly reported sensations were itching and tingling under the electrodes (Kuo et al., 2013; Caparelli-Daquer et al., 2012; Roy et al., 2014; Fresnoza et al., 2024). In studies that directly compared adverse effects across anodal, cathodal, and sham conditions, no significant differences were observed in participants’ ratings of pain or unpleasantness (Tabari et al., 2024), indicating similar tolerability across polarities. Other studies simply noted that participants tolerated both forms of stimulation well, with no adverse events reported (Stefano et al., 2022; Shen et al., 2016; Wang L. et al., 2024). Collectively, these findings suggest that stimulation polarity does not substantially affect tolerability, although further research is warranted to confirm this conclusion.
Overall, the consistency of these findings across different stimulation parameters and populations of healthy adults—including older adults—supports the tolerability and safety of HD-tDCS in non-clinical research settings. However, few studies have systematically investigated the tolerability of HD-tDCS, which has prevented the widespread application of it to the intervention of neurologic or psychiatric disorders.
Clinical populations
HD-tDCS has been used in a wide range of clinical populations including patients with fibromyalgia (Villamar et al., 2013a; Villamar et al., 2013b; Castillo-Saavedra et al., 2016), aphasia (Richardson et al., 2015; Fiori et al., 2019; Shah-Basak et al., 2020), chronic myofascial temporomandibular disorder pain (Donnell et al., 2015), tinnitus (Jacquemin et al., 2021; Jacquemin et al., 2018; Shekhawat et al., 2016; Shekhawat and Vanneste, 2018; Henin et al., 2016; Cardon et al., 2022), epilepsy (Karvigh et al., 2017; Ng et al., 2023; Roshan et al., 2024), schizophrenia (Sreeraj et al., 2018; Xu et al., 2023; Yeh et al., 2025), stroke (Bao et al., 2019; Hu et al., 2024), depression (Wong et al., 2019), traumatic brain injury (Motes et al., 2020; Chiang et al., 2021), chronic low back pain (McPhee and Graven-Nielsen, 2021), diabetes mellitus (Filipas et al., 2022), obsessive compulsive disorder (Wang Y. et al., 2024), Alzheimer’s disease (Lobue et al., 2025) and methamphetamine use disorder (Hou et al., 2025). Across these diverse samples, HD-tDCS has been shown to be generally well tolerated, with reported adverse events typically limited to transient sensations such as tingling, itching, or mild burning (Table 2). Importantly, no serious adverse effects have been reported in any of the reviewed studies, underscoring the safety profile of HD-tDCS in clinical contexts.
Table 2. Study design and adverse event reports for clinical populations.
Adverse events by stimulation parameters
In clinical populations, HD-tDCS has most commonly been administered at 2 mA (Villamar et al., 2013b; Castillo-Saavedra et al., 2016; Wong et al., 2019; Jacquemin et al., 2021; Jacquemin et al., 2018; Shah-Basak et al., 2020; Donnell et al., 2015; Shekhawat and Vanneste, 2018; Henin et al., 2016; Cardon et al., 2022; Karvigh et al., 2017; Ng et al., 2023; Roshan et al., 2024; Sreeraj et al., 2018; Xu et al., 2023; Yeh et al., 2025; McPhee and Graven-Nielsen, 2021), though some studies have used lower intensities of 1 mA (Richardson et al., 2015; Bao et al., 2019; Motes et al., 2020; Chiang et al., 2021) or 1.5 mA (Filipas et al., 2022; Wang Y. et al., 2024; Hou et al., 2025). Across all intensities, the most frequently reported sensations include tingling and itching, with occasional reports of burning (Jacquemin et al., 2021; Richardson et al., 2015; Sreeraj et al., 2018; Yeh et al., 2025; Lobue et al., 2025) and headache (Castillo-Saavedra et al., 2016; Jacquemin et al., 2021; Cardon et al., 2022; Karvigh et al., 2017; Sreeraj et al., 2018; Yeh et al., 2025). These effects are typically mild and transient. Several studies report no adverse effects at 1 mA (Motes et al., 2020; Chiang et al., 2021), 1.5 mA (Filipas et al., 2022), or even at 2 mA (Henin et al., 2016; Ng et al., 2023). In direct comparisons between intensities, one study involving patients with tinnitus found that tingling, sleepiness, and mild scalp discomfort were more frequently reported at 2 mA than 1 mA, though all reported sensations across conditions were brief and occurred primarily at the onset of stimulation (Shekhawat et al., 2016). Another study in patients with aphasia reported no significant differences in adverse effects between 1 and 2 mA stimulation (Fiori et al., 2019), and similar results were found in patients with Alzheimer’s disease, where mild to moderate sensations were observed across both intensities (Lobue et al., 2025).
While the majority of studies describe adverse effects as “mild” (Villamar et al., 2013a; Castillo-Saavedra et al., 2016; Wong et al., 2019; Shah-Basak et al., 2020; Karvigh et al., 2017; Roshan et al., 2024; Hu et al., 2024; Lobue et al., 2025), a few have reported moderate discomfort (Villamar et al., 2013a; Lobue et al., 2025) and in rare instances, more severe symptoms have been noted. For example, in one study applying 2 mA stimulation for 30 min to the right DLPFC and left temporal area using two 4 × 1 arrays (10 electrodes total), a single participant out of 77 with chronic subjective tinnitus experiences a more serious migraine-like headache (Cardon et al., 2022). This event occurred under a longer than typical 30 min protocol with multiple stimulation sites, suggesting that extended duration and multi-target montages may modestly increase the likelihood of adverse events, though additional research is needed to confirm this. Studies using intensities above 2 mA are limited, but one investigation using 4 mA in chronic stroke survivors over 20 sessions reported only minor side effects, with no significant difference in adverse event rates between active and sham conditions (Hu et al., 2024). These findings suggest that, with appropriate monitoring and protocol design, HD-tDCS may be safely administered at higher intensities in therapeutic contexts.
A critical consideration in clinical populations is whether HD-tDCS exacerbates existing symptoms. Encouragingly, the technique has generally been found to either improve clinical outcomes or have negligible negative impact. Reports of symptom aggravation are exceedingly rare. One study involving 117 patients with chronic tinnitus noted a single case in which symptoms were worsened (Jacquemin et al., 2021). Although uncommon, such findings underscore the importance of routine monitoring, particularly in multi-session treatment protocols, to ensure patient safety and responsiveness to the intervention.
Adverse events by number of HD-tDCS sessions
Unlike studies in healthy individuals, HD-tDCS research in clinical populations more commonly involves multi-session protocols. Although the number of sessions varies widely, from a single exposure to 20 or more sessions, repeated stimulation generally does not appear to increase the occurrence or severity of adverse events. Across studies administering 10 or more sessions, participants most often report only mild and transient sensations, such as tingling or itching (Castillo-Saavedra et al., 2016; Wong et al., 2019; Richardson et al., 2015; Karvigh et al., 2017; Roshan et al., 2024; Sreeraj et al., 2018; Hu et al., 2024; Lobue et al., 2025). For example, one study involving individuals with chronic traumatic brain injury found no significant change in self-reported pain or discomfort across 10 sessions, suggesting stable tolerability over time (Chiang et al., 2021). While rare, instances of early discontinuation due to discomfort have been documents. In one study, a participant with chronic low back pain discontinued stimulation on day three due to scalp discomfort, despite tolerating earlier sessions (McPhee and Graven-Nielsen, 2021). These findings suggest that HD-tDCS is well tolerated across repeated sessions in clinical populations, though individual variability in sensitivity should be monitored.
Adverse events by polarity: anodal vs. cathodal
Relatively few studies in clinical populations have directly compared the side effects associated with anodal versus cathodal HD-tDCS. However, existing evidence in studies employing both 3×1 and 4×1 ring montages suggests that polarity has minimal influence on the frequency or severity of adverse events. In patients with fibromyalgia, both anodal and cathodal stimulation produced similar mild-to-moderate sensations, such as tingling and itching (Villamar et al., 2013a). Comparable findings were reported in individuals with chronic stroke, where participants experienced transient scalp sensations, primarily tingling and itching under the central electrode, at the beginning and end of both stimulation conditions (Bao et al., 2019). Similarly, a study involving patients with aphasia noted only minor discomfort during both anodal and cathodal protocols (Shah-Basak et al., 2020). Although further research is needed to confirm these patterns across a broader range of clinical conditions, current findings indicate that HD-tDCS polarity does not substantially affect tolerability in clinical populations.
Exploratory analysis of AE frequency with stimulation parameters
Although most studies report only mild adverse events, it is equally important to consider research in which participants experience comparable stimulation parameters without any reported side effects. In both healthy and clinical populations, studies using 2 mA for 20 min consistently report no side effects (Shen et al., 2016; Xiao et al., 2020; Wan et al., 2021; Kold and Graven-Nielsen, 2021; Kold and Graven-Nielsen, 2022; Wang L. et al., 2024; Ng et al., 2023). In several of these studies participants experienced both active and sham conditions and still reported no adverse effects, effectively serving as their own controls and underscoring the robust tolerability of HD-tDCS (Martin et al., 2019a; Stefano et al., 2022; Flood et al., 2016; Shen et al., 2016; Xiao et al., 2020; Wan et al., 2021; Shen et al., 2024; Henin et al., 2016; Filipas et al., 2022). Additionally, a study administering multiple intensities (1 mA, 2 mA, 3 mA) in short blocks using a 3×1 montage targeting the right TPJ reported no adverse events (Stefano et al., 2022). These findings indicate that although mild adverse events may be more likely at higher intensities or longer durations, their relationship is not conclusive.
To explore whether higher stimulation parameters were associated with increased adverse event (AE) reporting, a simple exploratory analysis was conducted using data extracted from the reviewed studies. Each study was coded for the presence or absence of any reported AE or any significant group differences, and stimulation parameters were classified by intensity (≤1.5 mA vs. ≥2 mA), duration (≤20 min vs. >20 min), and polarity (anodal, cathodal, or both).
Among 33 studies in clinical populations, AE frequency was not significantly different across studies that used different stimulation intensity, duration, nor polarity (p > 0.18). When studies were compared that used different stimulation intensity, 54.5% of those using <2 mA reported at least one mild AE, compared to 77.3% of studies using ≥2 mA. This difference was not statistically significant (χ2(1) = 1.793, p = 0.181). When duration was examined, 100% of studies using <20 min reported at least one mild AE compared to 66.7% of those using ≥20 min (χ2(1) = 1.435, p = 0.231). When polarity was considered, 73.7% of anodal studies reported at least one mild AE, compared to 42.9% of cathodal studies and 85.7% of those using both polarities (χ2(1) = 3.381, p = 0.184).
Likewise, no significant differences were observed when AE reporting was compared across 80 studies in healthy populations (p > 0.32). 55.6% of those using <2 mA reported at least one mild AE, compared to 56.8% of studies using ≥2 mA (χ2(1) = 0.013, p = 0.910). Similarly, 47.4% of studies using <20 min reported at least one mild AE compared to 59% of those using ≥20 min (χ2(1) = 0.799, p = 0.371). When polarity was examined, 54.2% of anodal studies reported at least one mild AE, compared to 33.3% of cathodal studies and 65.4% of those using both polarities (χ2(1) = 2.247, p = 0.325).
Taken together, these findings indicate that within the commonly used stimulation ranges (1–2 mA, 10–30 min), HD-tDCS is well tolerated in both clinical and healthy participants, and parameter variations do not appear to systematically alter the likelihood of AE reporting.
Inconsistent adverse event reporting in the literature
Despite overall support for the tolerability of HD-tDCS, adverse event reporting remains inconsistent across both healthy and clinical populations. While some studies provide detailed accounts of stimulation related sensations, even if mild (Castillo-Saavedra et al., 2016; Jacquemin et al., 2021; Shah-Basak et al., 2020; Cardon et al., 2022; Sreeraj et al., 2018; Lobue et al., 2025), others merely state that the procedure was “well-tolerated” or that no adverse effects were reported (Ng et al., 2023; Motes et al., 2020; Filipas et al., 2022), without offering specifics or describing how side effects were assessed. A particularly concerning issue is that many studies describe stimulation parameters and experimental procedures in great detail but fail to mention adverse effects or participant tolerability altogether. In some cases it is stated that tolerability or symptom assessment was part of the methodology (Donaldson et al., 2018; Gallo et al., 2018; Chen et al., 2019; Gaynor and Chua, 2019; de Boer et al., 2020; Masina et al., 2021; Erker et al., 2024), such as by including an adverse event questionnaire or participant interviews, but these results are not reported in the findings or discussion.
In cases in which adverse events are reported, two main approaches are typically used. Some studies present adverse events on an individual basis, detailing the symptoms experienced by each participant and their perceived severity, while others summarize adverse events at the group level to compare active and sham stimulation conditions. Assessment methods also vary, from standardized questionnaires that quantify predefined adverse events to open-ended reports in which participants simply indicate whether they experienced any sensation. As a result of this heterogeneity, comparing the frequency and severity of adverse events across studies is challenging and is limited to general reports of sensations. This variability highlights the need for standardized adverse event assessment tools to enable more consistent comparisons across HD-tDCS research. Despite this variability, among the 39 studies reporting adverse events in Tables 1, 2, the most common sensations were tingling (85%), itching (69%), and burning (44%).
This lack of standardized adverse event reporting poses a significant challenge for comprehensive safety evaluation. The variability in language (e.g., “discomfort,” “pain,” “burning”) and the inconsistent timing of assessments (during stimulation, immediately post-session, or at follow-up) further complicates the interpretation and comparison of side effect profiles across clinical studies. Moreover, the use of different scales—or the absence of any scale—makes it difficult to gage the true incidence and impact of minor but clinically relevant sensations. These issues are especially concerning in the context of clinical populations, where HD-tDCS is used as a non-pharmacological alternative or adjunct to traditional treatments. Given that one of the main appeals of HD-tDCS in these groups lies in it being non-invasive and low-risk, systematic safety reporting is necessary to support its clinical adoption, regulatory approval, and patient confidence. The underreporting of adverse effects—even if mild or absent—can inadvertently undermine the very rationale for using HD-tDCS in these populations.
Additionally, the definitions for weak, mild, moderate and severe used in each study are typically based on participants’ subjective ratings, often collected with Likert-type or numeric rating scales, but the precise definitions are not always provided. This limits the extent to which direct comparisons can be made across studies. Despite this variability, the majority of reports converge in describing adverse events as mild and transient, with very few instances described as severe.
To advance the field, future studies should adopt standardized protocols for adverse event reporting. This includes predefining tolerability metrics including explicit definitions of severity levels, administering structured and validated side effect questionnaires—such as the tDCS adverse effects questionnaire proposed by Brunoni et al. (2011), and explicitly detailing both the presence and absence of side effects across all stimulation conditions. Comprehensive and systematic reporting will not only improve the interpretability and comparability of findings but will also help build a robust evidence base for HD-tDCS as a safe and effective tool for both research and clinical practice.
Conclusion
Across both healthy and clinical populations, HD-tDCS has consistently demonstrated a favorable safety and tolerability profile. The vast majority of reported adverse events are mild, transient and comparable to those observed with conventional tDCS, typically limited to sensations such as tingling, itching, or slight burning at the electrode sites. Serious side effects are exceedingly rare, and isolated reports of severe adverse events, such as burning, tingling, or headache, tended to occur under conditions involving higher current intensities, longer durations, or multi-site montages, and symptoms typically resolved quickly without lasting effects. Across a wide range of stimulation parameters and clinical conditions, HD-tDCS has demonstrated to be a well-tolerated, non-invasive technique, supporting its use as a safe and reliable tool for both research and therapeutic applications.
Author contributions
TC-K: Writing – original draft, Writing – review & editing. JK: Writing – review & editing.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. JK is supported by Canada Research Chair Program (CRC-2023-00214) and Natural Science and Engineering Research Council of Canada (RGPIN-2023-04283). TC-K is supported by a Postdoctoral fellowship sponsored by Parkinson Canada (PRF-2024-00000001390).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Correction note
This article has been corrected with minor changes. These changes do not impact the scientific content of the article.
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Keywords: HD-tDCS, safety, tolerability, adverse event, brain stimulation
Citation: Carther-Krone T and Ko JH (2025) A review on the reporting and assessment of adverse effects associated with high-definition transcranial direct current stimulation. Front. Hum. Neurosci. 19:1682771. doi: 10.3389/fnhum.2025.1682771
Edited by:
Zhen Yuan, University of Macau, ChinaReviewed by:
Bettina Habelt, Technical University Dresden, GermanyPedro José Tomaselli, University of São Paulo, Brazil
Copyright © 2025 Carther-Krone and Ko. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Ji Hyun Ko, amkua29AdW1hbml0b2JhLmNh