Repetitive transcranial magnetic stimulation for substance use disorders and chronic pain: A review of the evidence and call for increased mechanistic understanding.

Purpose of Review: Both substance use disorders (SUD) and chronic pain are highly prevalent, require new effective interventions, and share similar underlying neurocircuitry. A promising intervention for both is repetitive transcranial magnetic stimulation (rTMS) which is a tool to modulate brain circuits. In this narrative review, we surveyed the current state of rTMS used to address SUDs and chronic pain by focusing on 132 reports published since 2010. Recent Findings: The field agrees that rTMS application in these clinical samples is safe and potentially effective, even in co-occurring disease or with recent substance use. Overall, the pain field has come to a stronger consensus about how best to apply rTMS to reduce diverse chronic pain conditions compared to the SUD field. We argue for standardization of methods within fields to rapidly address each clinical group. Such a standardization will require a concerted effort to test applications head-to-head to evaluate relative efficacy across applications. A coordinated effort in this regard is needed between research groups and funding agencies to accomplish this goal. Summary: One of the most effective steps the field can take toward this goal is to measure the neuromechanisms that underlie SUD and pain before and after an rTMS intervention. Only 18% of reviewed reports included any brain measurement such as functional magnetic resonance imaging, electroencephalography, and event-related potentials. We argue that such measurements are essential for optimizing rTMS as an intervention for clinical populations. If rTMS is ostensibly applied to modulate neurocircuits, measurement of those circuits to verify, iterate, and optimize application is fundamental for developing effective treatments.