Ketamine as a Bridge to TMS: The Induction Protocol for Treatment-Resistant Depression

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The standard course of repetitive transcranial magnetic stimulation for treatment-resistant depression runs 20–36 sessions over four to six weeks. Response rates in well-selected patients are genuinely impressive — 50–60% by most prospective series — but the trajectory matters as much as the endpoint. TMS works through accumulated long-term potentiation across prefrontal-limbic circuits. The first session does nothing measurable. The first week does little. For many patients, the first meaningful shift in mood, sleep, or anhedonia does not arrive until week three or four.

For a patient with moderate-to-severe depression and stable support, this timeline is manageable. For a patient with active suicidal ideation, severe early agitation, or a history of previous treatment failures that have already cost them months of suffering, those first two to four weeks represent a real clinical risk. That is the window where the ketamine bridge has a rational, evidence-informed role.

This is not a strategy to use broadly. It is a protocol refinement for a specific clinical scenario — and one that must never slide toward positioning ketamine as the primary intervention. IV ketamine is a time-limited tool. The TMS is the destination.

Why TMS has an induction lag

Repetitive TMS at standard 10 Hz over the left dorsolateral prefrontal cortex (DLPFC) exerts its antidepressant effect primarily through synaptic strengthening in the frontal-striatal-limbic network. The mechanism is Hebbian: repeated co-activation of presynaptic and postsynaptic neurons drives AMPA receptor upregulation and dendritic spine growth, progressively strengthening connections that depression has weakened or silenced. This is the same mechanism as long-term potentiation (LTP) studied in hippocampal slices, now applied clinically.

The therapeutic timeline of LTP-based remodeling is measured in weeks, not hours. Neuroimaging studies of TMS responders show metabolic normalization in the left DLPFC, anterior cingulate cortex, and subgenual cingulate that accumulates across the treatment course, with the most reliable changes appearing after 15–20 sessions. First-session motor threshold measurements — the standard TMS calibration procedure — confirm that individual cortical excitability varies widely and that targeting must be adapted session by session.

For the patient sitting in the first week of TMS, none of this circuit remodeling has happened yet. They are receiving treatment that is working — in the sense that the biology is engaged — but the subjective experience of that work is not yet visible.

What ketamine does in that window

Ketamine blocks N-methyl-D-aspartate (NMDA) glutamate receptors. That block triggers a downstream cascade that matters far more than the initial mechanism: within hours, BDNF (brain-derived neurotrophic factor) is released in the prefrontal cortex, mTOR signaling activates, and new synaptic connections form at a rate that normal antidepressant timelines cannot match.

Murrough and colleagues randomized 73 patients with treatment-resistant major depression to a single IV ketamine infusion or active placebo (midazolam). At 24 hours, response — defined as a ≥50% reduction in depression scores — was 64% in the ketamine group versus 28% with midazolam (odds ratio 2.18; 95% CI 1.21–4.14; PMID 23982301). That effect is fast enough to change the clinical situation within a day of infusion.

The limitation of ketamine as a standalone treatment is equally important: the effect fades. Without a consolidating intervention, remission duration after a standard six-infusion course is typically two to eight weeks. Some patients require ongoing maintenance infusions. That recurring dependency is not an acceptable long-term model — not from a clinical standpoint, and not from Julian’s approach to depression, which targets circuit dysfunction rather than chemical suppression.

Ketamine does not fix the underlying prefrontal circuit architecture that leaves patients vulnerable to relapse. TMS does. The two interventions target different biology, on different timescales, and they can be sequenced deliberately to exploit both.

The bridge protocol — timing and dosing

The bridge protocol runs alongside, not before, the TMS course. Ketamine infusions begin on day one or two of TMS, with the goal of providing symptom stabilization while early LTP accumulates in the prefrontal circuits.

Standard bridge course:

• 3–6 IV infusions of ketamine at 0.5 mg/kg over 40 minutes
• Infusions delivered two to three times per week during weeks 1 and 2 of TMS
• TMS sessions proceed on schedule (five per week) throughout
• After the bridge course, TMS continues without ketamine through its full 20–36 session arc

The rationale for three to six infusions rather than a standard six-infusion depression course is practical: by week three, well-selected TMS patients begin to notice changes. At that point, extending the ketamine adds dependency risk without adding proportionate benefit. The goal was always stabilization, not independent antidepressant response.

The 2019 retrospective series by McLaughlin and colleagues (n=28, PMID 31440588) demonstrated an additional operational benefit of combined delivery: coincident ketamine administration allowed higher TMS output intensities than patients would tolerate with TMS alone. Higher intensities may translate to greater cortical penetration and faster LTP induction, though this needs prospective replication.

For patients where active suicidal ideation is the clinical driver — where the bridge is about survival through the induction period rather than comfort — intranasal esketamine (Spravato) is an alternative with a more controllable dosing schedule. The clinical logic is identical: short-term acute relief while TMS builds, with cessation once TMS response is established.

What the evidence shows about combined delivery

A 2024 Cureus systematic review (Arubuolawe et al., Cureus 16(7):e64712) pooled available studies on combined TMS and ketamine for treatment-resistant depression and found consistent case-level evidence of tolerability and plausibility, but no randomized trial establishing superiority over either alone.

A 2024 Psychopharmacology pilot by Shanok and colleagues compared 169 patients who received deep TMS alone versus 66 who received deep TMS plus six concurrent IV ketamine infusions. Response rates were 77% and 80% respectively; remission rates were approximately 40% and 43%. The difference was not statistically significant (DOI 10.1007/s00213-024-06573-1). The study was not randomized, and the groups were not matched, so the finding is best interpreted as “concurrent delivery does not appear to harm TMS outcomes” rather than “it adds no benefit” — patient selection differences between groups could easily mask a subgroup advantage.

For the question of efficacy parity among available interventions, Terao and colleagues published a 2025 network meta-analysis of 35 randomized trials (n=2,109) comparing IV ketamine, rTMS, and ECT for Stage 2 or higher treatment-resistant depression (PCN Reports 4(3):e70136, PMID 40590032). The analysis found no significant differences in response rate, remission rate, or tolerability among the three. IV ketamine was significantly more acceptable to patients than rTMS or ECT — a clinically meaningful finding for induction-phase decisions.

A 2023 mechanistic review in Frontiers in Neuroscience (PMC10637948) laid out the theoretical case for synergy: ketamine-driven BDNF release primes prefrontal synapses for the LTP that TMS subsequently drives. The two may not simply add their effects — ketamine may lower the LTP threshold that TMS must overcome.

This is the mechanistic framing that matters clinically. Ketamine does not make TMS do something it could not do alone. It may allow TMS to work faster, and it unambiguously extends the window in which high-risk patients can be safely maintained while TMS builds.

Who benefits from the bridge

The bridge protocol is not a first-line add-on for every TMS course. Clear indicators for considering it:

Primary indication — acute suicidal ideation during TMS induction. Patients entering TMS with active, persistent suicidal ideation represent the clinical scenario where the induction lag is least tolerable. A three-to-six infusion ketamine bridge provides the acute stabilization that allows TMS to proceed safely. This is not ketamine as a treatment of suicidality — it is bridge support for a patient who would otherwise be at unacceptable risk during the standard induction period.

Secondary indication — severe early anhedonia or agitation. Patients who have experienced multiple antidepressant failures may enter TMS in a state of profound anhedonia or early-morning agitation that makes the daily treatment attendance required for TMS logistically and psychologically very difficult. A short bridge course can provide enough lifting of symptoms to maintain adherence through the full course.

Tertiary indication — delayed previous TMS response. Some patients benefit from TMS but show first signs of response only after session 20 or 25. For these patients, re-entry into another TMS course with a bridge may shorten the time to functional improvement.

Who should not receive the bridge:

• Active or recent substance use disorder (especially alcohol or benzodiazepine dependence)
• Uncontrolled hypertension
• Personal or first-degree family history of psychosis or schizophrenia
• History of dissociative reactions to ketamine

Patients with a history of trauma or significant PTSD who also have depression should be evaluated individually — dissociation during ketamine infusion can be distressing and must be discussed in advance.

Monitoring and tapering — when to stop ketamine

The bridge should not extend beyond the first three to four weeks of TMS under any circumstances. The practical tapering protocol:

• Week 2 (sessions 6–10 of TMS): Complete the bridge infusion course.
• Week 3 (sessions 11–15): Assess early TMS response — PHQ-9, clinical interview, sleep, anhedonia, suicidal ideation. If response is emerging, no further ketamine.
• Week 4 (sessions 16–20): Confirm response trajectory. TMS continues to its full session count.
• If response not emerging at week 4: Revisit the diagnostic workup before considering further ketamine — this is the signal to look again for missed inflammation, occult infection, or relational drivers of the depression. See the treatment-resistant depression cause-first workup protocol for the full framework.

Extended ketamine use beyond this period — repeat infusions to maintain an effect that TMS has not yet produced — is not a bridge. It is a dependency. The whole rationale for the bridge model collapses if ketamine becomes the ongoing intervention.

Patients should be monitored with standardized scales (PHQ-9, MADRS or HAM-D) at weeks 2, 4, and 6. Ketamine dissociation should be recorded and graded for each infusion session. Blood pressure should be monitored during infusion.

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