Concentration is defined as sustained attention, or the ability to focus on a particular stimulus for an extended period ([27]). Concentration has two key elements: focusing on pertinent stimuli over time and filtering out distractions or irrelevant stimuli ([39]). Many people experience an inability to concentrate effectively which negatively affects their ability to carry out tasks in their daily lives; 15% of people living in the European Union (EU) report concentration issues ([12]). People may wish to enhance their ability to concentrate to better undertake tasks and achieve their goals, in an increasingly complex information and technology based society demands on cognitive functioning are high ([11]). People employ various biochemical, physical, and behavioural strategies to enhance their cognitive functioning; evidence for and possible negative effects of these strategies are required to enable people to make the best choices ([11]).

Concentration deficits may be attributable to a wide range of psychiatric and systemic illnesses ([30]). Problems with concentration are mentioned in the DSM-5-TR in relation to depressive disorders (major depressive disorder [MDD], dysthymia, and premenstrual dysphoric disorder), generalized anxiety disorder, bipolar disorder, post-traumatic stress disorder (PTSD), traumatic brain injury (TBI), acute stress disorder, dissociative identity disorder, sleep disorders (insomnia, normal sleep variation, hypersomnolence disorder, and nightmare disorder), stimulant use disorder, and attenuated psychosis syndrome ([2]). Cognitive impairment and concentration difficulties are features of unipolar depressive disorders, partially independent of the severity of other symptoms, and they can have a unique negative impact on quality of life ([14]). Cognitive deficits (including concentration problems) often persist despite reduction in depression, indicating need for interventions to address these problems ([37]).

Problems with concentration may be due to lifestyle factors ([27]). Lifestyle factors that can contribute to concentration problems include poor quality sleep, poor diet (high sugar intake, low nutrient intake, consumption of highly processed foods, and a lack of fresh food), lack of physical activity, cannabis use, and alcohol use ([28]). Dehydration can also negatively affect concentration ([16]).

Concentration problems can be caused by environmental factors that negatively affect sleep: noise, light exposure, disturbances from other people, temperature, and an uncomfortable bed ([18]). Wake-time environmental factors that can negatively impact concentration include excessively cold or hot temperatures and high humidity ([46]). Other factors include distractions due to noise, technological devices (e.g. mobile phones, computers), and other people ([47]).

Many people can experience concentration problems despite addressing lifestyle and environmental issues. Natural stimulants, such as ginkgo biloba and ginseng, may improve concentration, but not all people find them helpful, and effects may be temporary, with a subsequent drop in concentration following initial stimulation ([42]). Caffeine is widely used to enhance cognitive functioning and concentration ([9]). Caffeine can improve cognitive functioning and concentration, but negative effects of caffeine can include anxiety, jitteriness, and sleep problems, and at larger doses cardiac arrhythmias and tachycardia ([9]). Various food supplements have some evidence for improving concentration, including omega-3 fatty acids, iron and B vitamins (B12, B6, and B9); ensuring a diet contains sufficient amounts is advantageous ([3]; [24]).

Various forms of skills training are effective in enhancing cognitive functioning and concentration ([11]). Tactics include computer gaming, sudoku, crosswords, and musical activities ([11]). Specific ‘brain-training’ interventions can improve performance on the trained tasks, but there is less evidence that training improves everyday cognitive performance ([41]). A systematic review of strategy-based cognitive training specifically targeted at people experiencing depression revealed significant improvements in cognitive and affective outcomes ([49]).

Several drugs used for the treatment of attention deficit hyperactivity disorder (ADHD) are recreationally used to improve concentration and cognitive functioning; for example, drugs such as Adderall (mixed amphetamine salts) and Ritalin (methylphenidate) ([48]). Some people find that through taking such drugs they can improve cognition, including working memory, concentration, and memory consolidation, as well as increased wakefulness and motivation ([48]). However, such drugs can lead to addiction and physical dependence, cardiovascular problems (high blood pressure, increased heart rate), psychosis (hallucinations, paranoia), anxiety, and irritability, and there is potential for overdose and serious harm to health (with very rare cases of death) ([40]).

Modafinil is a central nervous system (CNS) stimulant eugeroic that directly increases cortical catecholamine levels, indirectly upregulates cerebral serotonin, glutamate, orexin, and histamine levels, and indirectly decreases cerebral gamma-aminobutyric acid levels ([4]; [33]). In healthy non-sleep-deprived individuals, modafinil can enhance attention, executive functions, and learning ([4]; [33]). Modafinil can improve executive function in currently depressed patients ([10]). Compared to other CNS drugs such as Adderall and Ritalin, Modafinil has a lower risk of dependency and fewer negative side effects, but these risk factors are still present ([17]).

Non-invasive brain stimulation (NIBS) techniques can improve cognition in people experiencing depression; for example, transcranial magnetic stimulation (TMS) can improve concentration, executive functioning, and working memory ([32]). Transcranial direct current stimulation (tDCS) is a type of NIBS that uses mild electric currents to modulate brain activity and treat neurological and mental disorders ([36]). tDCS devices are easy to use, have relatively low cost and can be used at home while maintaining an excellent safety and tolerability profile ([7]; [31]). tDCS treatment has only a few negative side effects ([7]); the most reported side effect is a mild headache lasting less than an hour ([36]). There is evidence for the use of tDCS in the effective treatment of depression ([6]; [23]; 2024a; 2024b; 2024c; 2025; [36]), anxiety ([38]; [43]), tinnitus ([50]), and pain ([26]). The tDCS protocol of bi-directional via anode (which increases cortical arousal) and cathode (which decreases cortical arousal) over left and right dorsolateral prefrontal cortex (DLPFC), F3 and F4 electrodes on a standard electroencephalogram (EEG) positioning, respectively, with stimulation at 2 mA, has been found to be safe and well-tolerated, with mild temporary physical sensations ([36]).

Research evidence shows that tDCS can enhance cognitive performance in healthy and clinical populations ([35]). A systematic review and meta-analysis revealed significant tDCS effects on various cognitive functions, specifically improvements in working memory, reaction time (RT), inhibition RT, flexibility RT, theory of mind RT, working memory accuracy, theory of mind accuracy and flexibility accuracy ([34]). tDCS can effectively enhance cognitive task performance ([34]), and the ability to concentrate is a factor in effectively engaging and performing many cognitive tasks.

tDCS may improve concentration problems, but there is a need for more evidence. This large-scale retrospective data analysis study investigated the impact of Flow FL-100 tDCS on the self-reported concentration problems of people self-reporting that they have depression. This study addresses the question of “what is the impact of tDCS on concentration problems as measured by the concentration question of the Montgomery-Åsberg Depression Rating Scale Self (MADRS-S) scale ([45])?” The study investigates the change in concentration problems over a period of ten weeks of tDCS use.

This large-scale retrospective data analysis study found that Flow FL-100 tDCS had a positive impact on the self-reported concentration problems of those self-reporting depression using the Montgomery-Åsberg Depression Rating Scale Self (MADRS-S) scale ([45]). This study adds to existing evidence of the significant positive effects of tDCS on various cognitive functions ([34]). In people who adhered to the minimum tDCS protocol for inclusion in the study, the proportion who moved from reporting concentration problems to not reporting concentration problems was 29.3% at week 1, 37% at week 2, 45.2% at week 3, 54.7% at week 6, and 57.4% at week 10. People can gain the most benefit from using tDCS when they adhere to the full recommended protocol and use tDCS for ten weeks (although there is less gain from six weeks to ten, so a treatment period of six weeks could be recommended).

This study found that Flow FL-100 delivered tDCS can be used by patients at home with the instructions and support provided at their primary care provider and by the Flow Neuroscience website and software application. GP primary care services are well-placed to deliver tDCS depression treatment as they seek to understand a patient’s individual circumstances, such as problems concentrating, and provide individualised support and treatment. GP primary care services have successfully offered tDCS for depression with clinically significant improvement rates of over 50% ([23]; [22]). This study’s results suggest that tDCS works relatively quickly, which is aligned with other tDCS studies that reported positive results after only a few tDCS sessions ([1]; [8]; [15]). Therefore, tDCS could be considered as a treatment when a relatively quick relief of concentration problems is a key goal.

There was a clear association between depression and concentration problems in this study, and the link between depression and cognitive impairment is well documented ([14]). This emphasises the importance of seeking to reduce depression to improve concentration and cognitive functioning. Addressing cognitive impairment in addition to depressive symptoms is important in improving functional outcomes for people with depression ([37]). In improving depression and reducing concentration problems, tDCS contributes to the reduction in impaired real world functioning (work, domestic activities, social activities, leisure activities, social relationships) seen in other studies ([23]; 2024a; 2024b; 2024c; 2025).

Cognitive and concentration problems are a feature of mental health diagnoses other than depression ([2]). This study found that there was a reduction in concentration problems when users reported that they had an additional mental health diagnosis (bipolar disorder, PTSD, anxiety, ADHD), indicating the potential positive impact of tDCS where these diagnoses are the only or primary diagnosis. Concentration problems are a core feature of ADHD and, while the reduction in concentration problems was less in the group reporting this diagnosis it was still around 50% at week six and ten.

tDCS leads to modulations of all core components of executive functioning enabling assumptions about the cognitive mechanisms underlying these tDCS-related modulations ([44]). The potential neurobiological mechanisms through which tDCS might improve concentration could be investigated further. Future studies should seek to have an extended follow-up data collection period, to help determine longer term effects of tDCS and if “top up” treatments might be useful or required to sustain any benefits. Future studies could investigate tDCS as a method of reducing concentration problems where they occur without the presence of depression.

The study had several limitations. The use of Flow FL-100 tDCS was “open-label” and adjunct to any existing other treatments or therapies and unmeasured factors such as lifestyle changes, which could influence depression and concentration. All data were self-reported. This study relied on a single concentration question (MADRS-S question 5). Future studies should incorporate standardised, valid and reliable cognitive function assessments (e.g., The Perceived Deficits Questionnaire-Depression (PDQ-D), [25] and the THINC-Integrated Tool (THINC-it) screening assessment for cognitive dysfunction [[29]]). There can be a placebo effect of tDCS on cognitive functioning ([5]), but in this study, tDCS was not being given for improving cognitive functioning (it was given for addressing depression), and participants were not told that tDCS may improve concentration. The participant population was people who choose to buy Flow FL-100 for themselves or who had been given a Flow FL-100, this population may differ from broader clinical population with depression in terms of severity of depression, current treatment, sociodemographics, motivation, socioeconomic status, and treatment history.

This study is the first to report that greater adherence to the tDCS protocol and longer use are associated with greater improvement in the proportion of people reporting no concentration problems. tDCS can be used for a relatively quick reduction in concentration problems. It is important to be able to provide people with depression a treatment option for reducing their concentration problems. While tDCS treatment may have reduced concentration problems symptoms for some, it is unclear why some patients did not respond. Research is needed to understand individual differences in response, particularly regarding the mechanisms of tDCS action.

Flow tDCS can be purchased and used independently of healthcare services in several countries; however, its cost is prohibitive for many people, and awareness and availability within healthcare systems are low. Improving access through free universal healthcare systems like the UK’s NHS would help address treatment inequality. Primary care general practices are well-placed to prescribe tDCS treatment as most people first seek help for depression through a GP, and many patients are treated by their GP for long-term or recurrent depression. Appropriately designed and powered RCTs are warranted to investigate the impact of tDCS on cognitive functioning and concentration.

Funding for the study was provided by Flow Neuroscience AB; however, the study’s design, analysis, interpretation, and reporting were conducted independently of their involvement.