Treatment-resistant depression (TRD) is commonly defined as the failure to achieve an adequate clinical response following at least two trials of antidepressant therapies administered at appropriate doses and durations.¹ Although definitions vary across studies and clinical guidelines, TRD represents a substantial and persistent challenge in the management of major depressive disorder. Epidemiological estimates suggest that approximately 20%–30% of individuals with depression develop treatment resistance, highlighting the limitations of current first- and second-line interventions.²

The burden of TRD is profound and multifaceted. Individuals with TRD experience more severe and chronic depressive symptoms, higher rates of relapse and recurrence, and significantly greater functional impairment compared with treatment-responsive patients.³ TRD is strongly associated with increased risk of suicide, comorbid anxiety and substance use disorders, cognitive dysfunction, and diminished quality of life.⁴ From a societal perspective, TRD contributes disproportionately to health-care utilization, including frequent hospitalizations, emergency presentations, and long-term pharmacotherapy, as well as indirect costs related to lost productivity, disability, and caregiver burden.⁵

Despite the availability of multiple pharmacological agents and psychotherapeutic approaches, clinical outcomes for TRD remain suboptimal. Conventional antidepressants often yield modest incremental benefits with each successive trial, while cumulative side effects, delayed onset of action, and poor tolerability further limit adherence and effectiveness.⁶ Although electroconvulsive therapy remains the most efficacious intervention for severe and refractory depression, its use is constrained by stigma, cognitive adverse effects, limited access, and patient acceptability.^7,8 Consequently, many individuals with TRD cycle go through repetitive treatment strategies with diminishing returns, underscoring a significant unmet need for interventions that are both effective and acceptable.^9,10 These unmet clinical needs extend beyond symptom reduction to include treatments capable of producing rapid, durable responses, improving functional recovery, and addressing the neurobiological heterogeneity underlying TRD.^1,2 Moreover, there is a growing demand for personalized, mechanism-driven therapies that move beyond the traditional monoaminergic framework of depression.¹¹

Addressing these gaps is critical to reducing the individual and societal burden of TRD and has driven increasing interest in innovative approaches such as psychedelic-assisted therapies and neuromodulation, which are explored in this review.¹² Pharmacotherapy and psychotherapy remain the cornerstones of treatment for major depressive disorder; however, their effectiveness diminishes substantially in TRD. With each successive antidepressant trial, the likelihood of achieving remission decreases—a phenomenon well documented in large pragmatic studies.^3,6 Many commonly prescribed antidepressants share overlapping monoaminergic mechanisms of action, which may limit their utility in patients whose illness is driven by alternative neurobiological pathways.¹³ As a result, repeated medication switching or augmentation often yields only modest symptomatic improvements while increasing the risk of cumulative adverse effects.

Pharmacological treatments are further constrained by the delayed onset of therapeutic action, typically requiring several weeks before meaningful symptom improvement is observed. For individuals with TRD—who frequently experience severe, chronic, or suicidal symptoms—this delay poses significant clinical risk.¹⁰ Additionally, antidepressant medications are associated with side effects, including sexual dysfunction, weight gain, sleep disturbances, emotional blunting, and gastrointestinal symptoms, which can compromise adherence and long-term tolerability.^6,13 Polypharmacy, commonly employed in TRD, further increases the risk of drug–drug interactions and metabolic burden without guaranteed benefit. Psychotherapy, including evidence-based modalities such as cognitive behavioral therapy, interpersonal therapy, and psychodynamic approaches, plays a critical role in comprehensive depression care but has limitations in TRD. While psychotherapy can improve coping strategies, emotional regulation, and functional outcomes, its antidepressant effects may be insufficient in individuals with severe or biologically entrenched illness.^1,3 Engagement in psychotherapy requires sustained motivation, cognitive capacity, and access to trained providers—factors that may be compromised in chronic depression or under-resourced settings. Moreover, psychotherapy typically produces gradual improvements, which may not meet urgent clinical needs.

This lack of precision contributes to prolonged periods of ineffective treatment, patient frustration, and therapeutic disengagement.^14,15 Collectively, these limitations underscore the need for novel interventions that offer faster onset, greater efficacy in refractory cases, and mechanisms extending beyond traditional monoaminergic and cognitive frameworks.¹¹

The limitations of conventional treatments have accelerated the search for innovative therapeutic approaches capable of addressing TRD’s clinical and neurobiological complexity. In this context, psychedelic-assisted therapy and neuromodulation have emerged as two of the most promising interventions in contemporary psychiatry. Both approaches represent a departure from traditional symptom-focused treatment models, targeting neural circuitry, neuroplasticity, and maladaptive cognitive-emotional patterns implicated in treatment resistance.^12,16

Psychedelic-assisted therapy involves the administration of psychoactive compounds—such as ketamine, psilocybin, and MDMA—within structured clinical settings incorporating preparation, guided dosing sessions, and post-session integration therapy. Unlike conventional antidepressants, which require daily administration over weeks, psychedelics often involve a limited number of sessions with rapid antidepressant effects.³ Evidence suggests these interventions may facilitate enduring changes in mood and cognition through enhanced neuroplasticity, modulation of default mode network activity, and emotionally salient experiences that catalyze therapeutic insight.^13,16

Neuromodulation techniques offer nonpharmacological methods of directly influencing dysfunctional brain networks. Advances in repetitive transcranial magnetic stimulation (rTMS), electroconvulsive therapy (ECT), deep brain stimulation (DBS), and magnetic seizure therapy (MST) illustrate a paradigm shift toward circuit-based models of depression.^14,15,17 These interventions aim to restore functional connectivity and neural balance through targeted stimulation rather than systemic pharmacological effects.

Despite their promise, both psychedelic-assisted therapies and neuromodulation raise important clinical, ethical, and implementation considerations. Questions remain regarding long-term efficacy, optimal patient selection, standardization of treatment protocols, safety monitoring, regulatory oversight, and scalability within diverse health-care systems.^18,19,20 The rapidly expanding literature on these interventions is heterogeneous, encompassing varying study designs, definitions of treatment resistance, outcome measures, and follow-up durations, making traditional systematic review synthesis challenging.

The primary objectives of this scoping review are to map current clinical and translational evidence on psychedelic-assisted therapies and neuromodulation for TRD, identify prevailing research trends, and highlight critical knowledge gaps. This review seeks to examine evidence balance across intervention modalities, assess consistency in outcome reporting and safety evaluation, and identify underexplored populations and implementation considerations, ultimately informing research priorities, clinical guidance, and policy development. In line with evolving academic standards, this manuscript adheres to the TITAN 2025 reporting guideline for the use of artificial intelligence in scientific writing and research transparency.²¹

Studies were included if they reported on adult populations with TRD and evaluated psychedelic-assisted therapies or neuromodulation interventions within a clinical or research context. Case reports involving fewer than three patients, non-peer-reviewed articles, conference abstracts, editorials, commentaries, and preclinical or animal studies were excluded. These restrictions were applied to ensure the inclusion of methodologically robust and clinically relevant evidence.

Study selection followed a two-step screening process to ensure methodological rigor and reproducibility.

The study selection process was documented using a PRISMA-ScR flow diagram (Figure 1), detailing the number of records identified, duplicates removed, articles screened, full-text articles assessed for eligibility, and studies included in the final review. Reasons for full-text exclusion were provided to ensure transparency.

Data charting facilitated the identification of several critical gaps in the current literature on treatment-resistant depression:

Limited long-term outcome data:

Many studies, particularly those investigating psychedelic-assisted therapies and ketamine, focused on short-term efficacy, with relatively few assessing sustained outcomes beyond 6–12 months.

Heterogeneity in definitions of TRD:

Inconsistent operationalization of treatment resistance across studies limited cross-study comparability and hindered meta-analytic synthesis.

Underrepresentation of specific populations:

Older adults, individuals with comorbid psychiatric or medical conditions, and ethnically diverse populations were frequently underrepresented.

Insufficient head-to-head comparisons:

Few trials directly compared emerging interventions (e.g., psychedelics vs. neuromodulation or ketamine vs. TMS), limiting conclusions regarding relative efficacy and tolerability.

Variability in outcome measures:

Differences in primary endpoints and reliance on clinician-rated scales often overlooked patient-centered outcomes such as quality of life and functional recovery.

Mechanistic and Biomarker Gaps:

Despite growing interest in circuit-based and neuroplasticity-driven models, relatively few studies incorporated neuroimaging, electrophysiological measures, or biomarkers to guide treatment personalization.

Implications for Future Research

The identified gaps underscore the need for:

Addressing these limitations was essential for translating emerging treatments into scalable, evidence-based clinical care models.

Across modalities, interventions were safe and well-tolerated when delivered in controlled clinical settings. Adverse events were typically transient, mild to moderate in severity, and predictable (e.g., headache, nausea, transient blood pressure changes). Nonetheless, long-term safety data remain limited, particularly for newer psychedelic and neuromodulation approaches.^4,13

Collectively, these findings indicate that psychedelic-assisted therapies, ketamine, and neuromodulation represent promising strategies for TRD, offering rapid, durable, and mechanistically diverse treatment options. However, further research is needed to optimize maintenance, compare interventions head-to-head, and establish long-term safety profiles.

Study characteristics and outcomes were summarized in structured tables to enable cross-study comparison. Tables included:

This tabular synthesis highlighted differences in methodological rigor, follow-up duration, and outcome prioritization across intervention categories, while also clarifying areas of convergence in clinical benefit.

Visual mapping techniques were employed to illustrate trends and gaps within the literature. Conceptual figures depicted:

These visual representations facilitated identification of clusters of robust evidence as well as domains requiring further investigation, supporting a systems-level understanding of how emerging interventions may be integrated into future models of care.

Overall, the synthesis indicates that both psychedelic-assisted therapies and neuromodulation represent promising and complementary approaches for treatment-resistant depression. While rapid symptom relief and neuroplastic effects are common across modalities, variability in durability, accessibility, and mechanistic understanding underscores the importance of personalized, circuit-informed treatment strategies.

A total of 30 studies were included, encompassing randomized controlled trials (RCTs), open-label trials, and systematic reviews/meta-analyses published between 2010 and 2025. Psychedelic-assisted interventions (psilocybin, MDMA, ketamine) were investigated predominantly in early-phase RCTs for treatment-resistant depression (TRD) and post-traumatic stress disorder (PTSD), with participants ranging from young to older adults.¹¹ Neuromodulation studies included repetitive/transcranial magnetic stimulation (TMS), theta-burst stimulation (TBS), electroconvulsive therapy (ECT), and deep brain stimulation (DBS), mainly in adult TRD populations.²⁰ Study designs for psychedelics comprised approximately 50%–60% RCTs, 20%–25% open-label trials, and 15%–20% reviews; neuromodulation studies were 40% RCTs, 30% open-label, and 20% systematic reviews.

This review highlights the evolving landscape of treatment-resistant depression (TRD), demonstrating that both psychedelic-assisted interventions (psilocybin, MDMA, ketamine-assisted therapy) and neuromodulation techniques (TMS, TBS, ECT, DBS) offer clinically meaningful improvements in symptomatology (Table 3). Psychedelic therapies exhibit rapid onset of antidepressant effects, with sustained durability in controlled settings, complementing neuromodulation approaches that show moderate-to-high efficacy with well-characterized safety profiles.^11,12 These findings reinforce the paradigm shift from conventional monoaminergic pharmacotherapy toward mechanistically diverse interventions capable of targeting resistant depressive phenotypes.

Psychedelics, when administered with structured psychotherapy, offer rapid, robust, and durable antidepressant responses, presenting opportunities to reduce the latency and burden of conventional treatments.^16,22 Neuromodulation provides a non-pharmacological alternative with adaptable protocols (e.g., individualized TMS targeting, DBS for refractory cases), offering avenues for personalized care.^14,15 Together, these modalities could redefine TRD management, allowing stratification of patients based on clinical, neurobiological, and tolerability profiles.

While both approaches show promise, implementation faces ethical and societal challenges. Psychedelic-assisted therapy requires highly supervised clinical environments, raising questions about equitable access, scalability, and cultural acceptability.^18,19 Neuromodulation is limited by resource-intensive infrastructure (e.g., ECT requiring anesthesia, TMS requiring specialized equipment), which may restrict availability in low-resource settings. Furthermore, underrepresentation of diverse populations in clinical trials underscores the need for culturally sensitive recruitment and intervention strategies to ensure generalizability and global applicability.

The evidence base is limited by small sample sizes, heterogeneous designs, and short follow-up durations, particularly for psychedelic studies. Head-to-head trials are scarce, and neuromodulation studies are often site-specific with variable protocols. Long-term safety, cost-effectiveness, and integration with standard pharmacotherapy require additional investigation. Finally, this review synthesized published literature without quantitative meta-analysis; reported outcomes are pooled or approximated, introducing potential bias.