The rising trend of depression and its adverse effects have mobilized clinicians and researchers to investigate and explore various treatments. The first-line treatment for this psychiatric disability is therapy and antidepressant medication. However, nearly 50% of depression patients don’t see improvements following these interventions. 
What Is Transcranial Direct Current Stimulation?
Transcranial Direct Current Stimulation is a stimulation technique that delivers low electric current to the brain. It has shown promising results for modulating cognitive skills and improving psychiatric disorders. 
How Does tDCS Work?
tDCS sends a tiny electric current into the brain – typically 1-2 mA only. The safe maximum strength of 0.002 amperes is a tiny fraction of household electric current, but it is enough to change neural interactions. (Source: Frontiers)
The human brain has billions of neurons that communicate through electrochemical pulses. The interruption by tDCS affects these electrochemical pulses in the brain, changing brain function.  This makes tDCS an effective neuroscientific tool for psychiatric disorders.
Transcranial Direct Current Stimulation (tDCS) For Mental Health
tDCS may offer many benefits, including the following:
TDCS is a non-invasive procedure that heals without any cognitive damage as compared to brain surgeries.
- Improves Learning Ability and Skills Acquisition
Research suggests that Transcranial Direct Current Stimulation can improve learning ability and skills training.
A study found that applying tDCS helped participants memorize symbols and improved number processing and numerical abilities. The effects lasted up to six months post initial treatment. 
Another study of 104 subjects reported that tDCS improved learning when applied to the right inferior frontal and right parietal cortex.
- Can Reduce Pain
tDCS can be used to treat chronic pain conditions such as fibromyalgia.
- Reduces Depression Symptoms
One study of 22 patients suffering from major depressive disorder (MDD) found that applying anodal stimulation to the dorsolateral prefrontal cortex for two weeks enhanced positive emotions compared to the control placebo group.
- Improves ADHD Symptoms
Research shows that tDCS can potentially alleviate the symptoms of core ADHD.
- Improves Cognitive Function
Moreover, tDCS can also help in preventing cognitive decline.
For example, a study of eight patients with Parkinson’s showed that tDCS on the primary motor and premotor cortex improved their overall walking speed and balance (two motor behaviors often severely disrupted by Parkinson’s disease).
How does tDCS Help In Depression?
Nearly 80% of depression patients have a recurrence of depressive symptoms post-treatment with antidepressant drugs. 33% do not even get fully cured after 2 or 3 medication trials. (Source: NCBI)
Thus, the complexity, heterogeneity, and neurobiology of MDD – with etiology, symptoms, course, and response to the treatment – suggest the use of experimental techniques to influence the brain circuits and areas connected with this pathology.
Therefore, many psychologists are now showing interest in tDCS. Moreover, there is significant evidence showing the efficacy of tDCS for the treatment of depression.
The first studies of tDCS in depression (conducted more than ten years ago) reported promising results.
Another study of 2 placebo-controlled groups of hundreds of depression patients published in JAMA Psychiatry explored the role of tDCS for pharmacological therapies. The study found that tDCS combined with sertraline is more effective for the treatment of depression. 
Most meta-analyses have found that tDCS is a superior treatment for MDD than repetitive transcranial magnetic stimulation (rTMS) and antidepressant medication. (Source: NCBI)
tDCS can alleviate symptoms of depression indirectly in the following ways:
- Affects Food and Drug Cravings
A study reported that tDCS on the prefrontal cortex reduced sugar and carb cravings.
- By Changing Neurochemical Interactions
Many mental disorders are caused due to chemical imbalance in the brain [12, 13]. Applying tDCS to specific brain regions can help change neurochemical interactions in the brain, alleviating depression and other psychiatric disorders. 
Is tDCS FDA Approved?
tDCS is not yet FDA-approved, but it is approved for treating depression across much of the world, including Europe. (Source: FDA News)
The latest CANMAT (Canadian Network for Mood and Anxiety Treatments) edition and a European consensus of experts graded tDCS a popular treatment of depression.
Many neurologists and psychologists now use tCDS to treat psychiatric disabilities. For example, doctors at Johns Hopkins Physical Medicine and Rehabilitation use brain stimulation for traumatic brain injury (TBI), stroke, and several other conditions. It is also used for addressing symptoms related to depression, language disorders, movement disorders, impaired cognition, and chronic pain. (Source: John Hopkins Medicine)
- Treatment-resistant depression: therapeutic trends, challenges, and future directions by Khalid Saad Al-Harbi. 2012. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3363299/
- The sertraline versus electrical current therapy for treating depression clinical study: results from a factorial, randomized, controlled trial by Brunoni AR, Valiengo L, Baccaro A. 2013. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/23389323/
- Neurocognitive effects of Transcranial Direct Current Stimulation (tDCS) in unipolar and bipolar depression: Findings from an international randomized controlled trial by Shawn M McClintock. 2020. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/31944487/
- Excitability changes induced in the human motor cortex by weak Transcranial Direct Current Stimulation by M A Nitsche and W Paulus. 2000. Retrieved by https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2270099/
- Transcranial Direct Current Stimulation: a roadmap for research, from mechanism of action to clinical implementation by Henry W Chase, Megan A. Boudewyn, Cameron S. Carter, Mary L. Phillips. 2020. Retrieved from: https://escholarship.org/content/qt4zv154tq/qt4zv154tq_noSplash_e36cfc0130483654ed9db078f357030c.pdf
- Modulating neuronal activity produces specific and long-lasting changes in numerical competence by Roi Cohen Kadosh, Sonja Soskic, Teresa Iuculano, Ryota Kanai, Vincent Walsh. 2010. Retrieved from: https://www.ncbi.nlm.nih.gov/pubmed/21055945
- Anodal transcranial direct current stimulation of the prefrontal cortex enhances working memory by Felipe Fregni. 2005. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/15999258/
- Improving working memory: exploring the effect of transcranial random noise stimulation and transcranial direct current stimulation on the dorsolateral prefrontal cortex by Paul G Mulquiney, Kate E Hoy, Zafiris J Daskalakis, Paul B Fitzgerald. 2011. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/21665534/
- Improving working memory: the effect of combining cognitive activity and anodal transcranial direct current stimulation to the left dorsolateral prefrontal cortex by Sophie C Andrews, Kate E Hoy, Peter G Enticott, Zafiris J Daskalakis, Paul B Fitzgerald. 2011. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/21511208/
- Direct current stimulation over V5 enhances visuomotor coordination by improving motion perception in humans by Andrea Antal. 2004. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/15165345/
- Enhancing long-term memory with stimulation tunes visual attention in one trial by Robert M G Reinhart, Geoffrey F Woodman. 2015. Retrieved from: https://pubmed.ncbi.nlm.nih.gov/25548192/
- Chemistry of depression: https://www.verywellmind.com/the-chemistry-of-depression-1065137
- Effects of a chemical imbalance causal explanation on individuals’ perceptions of their depressive symptoms by Joshua J Kemp, James J Lickel, Brett J Deacon. 2014. Retrieved from:https://pubmed.ncbi.nlm.nih.gov/24657311/
- Transcranial direct current stimulation for major depression: A general system for quantifying transcranial electrotherapy dosage by Marom Bikson and colleagues. 2008. Retrieved from: https://link.springer.com/article/10.1007%2Fs11940-008-0040-y
- Transcranial Direct Current Stimulation (tDCS): https://www.hopkinsmedicine.org/psychiatry/specialty_areas/brain_stimulation/tDCS.html
- Can Zapping Your Brain Really Make You Smarter? https://daily.jstor.org/can-zapping-your-brain-really-make-you-smarter/
- Excitability changes induced in the human motor cortex by weak Transcranial Direct Current Stimulation by M A Nitsche and W Paulus. 2000. Retrieved from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2270099/
- Transcranial Direct Current Stimulation (tDCS) for unipolar and bipolar depression: https://www.nationalelfservice.net/mental-health/depression/transcranial-direct-current-stimulation-depression/
- What is Transcranial Direct Current Stimulation?: https://neuromodec.org/what-is-transcranial-direct-current-stimulation-tDCS/