Bipolar depression is a long-lasting and disabling condition, and many people continue to experience depressive symptoms despite standard treatments. Transcranial direct current stimulation (tDCS) is a non-invasive form of brain stimulation that uses a very small electrical current applied through the scalp and has shown promise as a treatment for depression. This study aims to find out whether a home-based tDCS device is effective and safe in reducing symptoms of bipolar depression when compared with a placebo (sham) treatment. The study will also look at how acceptable the treatment is to participants and how well people are able to use the device at home.

Who can participate? Adult patients aged 18 years and over who have a diagnosis of bipolar disorder and are currently experiencing a depressive episode.

What does the study involve? Participants must meet specific eligibility criteria, which will be assessed by the research team. People who do not meet the study criteria or for whom tDCS is not suitable will not be able to take part. Participants will be randomly assigned to receive either active tDCS or a placebo (sham) treatment. Neither the participant nor the researchers assessing outcomes will know which treatment has been assigned.

Participants will use a study device at home over a defined treatment period and will complete a series of assessments at set time points. These include clinician-rated interviews and self-reported questionnaires about mood and well-being. Device use and adherence data will be collected electronically. Participants will also be monitored for any side effects throughout their involvement in the study.

Although the study is multi-site, participation is primarily remote, with most study activities completed from the participant's home.

What are the possible benefits and risks of participating? Participants may experience an improvement in depressive symptoms. Information gained from this study may help improve future treatments for bipolar depression.

tDCS is generally well tolerated. Possible side effects include mild and temporary sensations such as tingling, itching, headache, or skin irritation at the electrode sites. All participants will be monitored for adverse events, and appropriate support will be available if needed.

Where is the study run from? The study is run from King's College London in collaboration with NHS research sites across the UK.

When is the study starting and how long is it expected to run for? April 2026 to October 2027

Who is funding the study? The National Institute for Health and Care Research (NIHR), UK.

Who is the main contact? Professor Cynthia Fu, the Chief Investigator at King's College London, cynthia.fu@kcl.ac.uk

Inclusion Criteria:

1. Adults aged 18 years or over.
2. Diagnosis bipolar disorder in a current depressive episode based on Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria assessed by structured clinical assessment, Mini-International Neuropsychiatric Interview (MINI) (Sheehan et al.,1998).
3. Having at least a moderate severity of depressive symptoms as measured by a score of at least 18 in MADRS (Montgomery and Åsberg, 1979).
4. Either not taking antidepressant medication or taking a stable dose of antidepressant medication for at least 6 weeks before enrolment.
5. Either not currently in psychotherapy or in ongoing psychotherapy for at least 6 weeks before enrolment.
6. Being under care of GP.
7. Agreeable for GP to be regularly informed by research team about participation.
8. Able to provide written, informed consent.

Exclusion Criteria:

1. Significant suicide risk as measured by answering 'yes' to questions 4, 5 or 6 on the Columbia Suicide Severity Rating Scale (C-SSRS) Screen (Posner et al., 2011).
2. Primary comorbid psychiatric disorder (e.g. obsessive compulsive disorder) based on DSM criteria as assessed in MINI (Sheehan et al., 1998).
3. Having a Young Mania Rating Scale (Young et al., 1978) score of 20 or more.
4. Current daily use of medications that affect cortical excitability (e.g. benzodiazepines).
5. Current illicit drug use or heavy alcohol use with high risk of alcohol use disorder as measured by a score of > 8 in Alcohol use disorders identification test consumption (AUDITC) (Khadlesari et al., 2017; NICE, 2023).
6. History of electroconvulsive therapy (ECT), transcranial magnetic stimulation (TMS), cranial electrotherapy stimulation (CES), transcranial direct current stimulation (tDCS), deep brain stimulation (DBS), other brain stimulation, or psychosurgery for depression.
7. History of esketamine / ketamine for treatment of depression.
8. Medical disorder that may mimic mood disorder (e.g. hormonal disorder).
9. History of myocardial infarction, coronary artery bypass graft (CABG), coronary heart failure (CHF), or history of other cardiac issues.
10. Have cognitive impairment (e.g. dementia).
11. History of a neurological disorder (e.g., cerebrovascular events, stroke, structural lesion, epilepsy, seizures, Parkinson's disease).
12. History of migraines or intractable headaches.
13. Implant in brain, neurocranial defect or active implantable medical device.
14. Shrapnel or any ferromagnetic material in head.
15. If female and of child-bearing potential, currently pregnant or planning to become pregnant during the study
16. Concurrent enrolment in another interventional study.

Deidentified individual participant data and the data dictionary will be made available on request to academic researchers. The Statistical Analysis Plan is available in the Supplementary Materials. A data request and brief analysis plan will be required in accordance with Ethics Committee requirements. These will be reviewed by the lead, study steering committee and study sponsor. A data transfer agreement (DTA) will have to be completed prior to any data being shared. Following completion of the DTA, data will be shared as password-protected files. Data sharing will abide by the rules and policies defined by the sponsor, relevant institutional review boards, and local, state and federal laws and regulations. Rights and privacy of individuals participating in the research will be protected at all times. Approval will not be provided for commercial use of the data.