Transcranial magnetic stimulation (TMS) uses rapid magnetic pulses delivered through the skull to generate small electrical currents inside the brain, activating neurons without surgery, anesthesia, or electrodes. It’s most commonly used to treat depression that hasn’t responded to medication, and it works by stimulating underactive brain circuits involved in mood regulation. The process is painless enough that you sit in a chair, stay fully awake, and drive yourself home afterward.
The Physics Behind the Pulse
TMS operates on a principle from basic physics called electromagnetic induction. A coil held against your scalp fires a brief, powerful magnetic pulse, typically between 1 and 4 Tesla (roughly the strength of an MRI machine) lasting less than a millisecond. That rapidly changing magnetic field passes through your skull and induces a small electric field in the brain tissue underneath. This electric field drives tiny currents carried by ions in and around neurons.
The currents don’t affect the brain evenly. They have their greatest impact at points where nerve fibers bend or where different parts of a neuron meet, such as where a long fiber connects to the cell body. These geometric “discontinuities” are where stimulation is most effective at triggering a neuron to fire. Because the magnetic field weakens with distance, TMS primarily reaches the outer layer of the brain (the cortex), penetrating roughly 1.5 to 3 centimeters beneath the coil depending on the device used.
What Happens Inside the Brain
When a TMS pulse forces neurons to fire, it sets off a cascade of chemical signaling. Neurons communicate by releasing neurotransmitters, the two most relevant being glutamate (which excites neighboring neurons) and GABA (which quiets them). TMS essentially tips the balance between these signals in the targeted area. The stimulated neurons also send signals along their natural connections to other brain regions, so a pulse delivered to one spot on the surface can influence deeper networks involved in mood, attention, or motor control.
A single pulse causes a brief, temporary effect. But when pulses are delivered repeatedly over many sessions, something more durable happens. Repetitive TMS appears to harness the same process the brain uses to strengthen or weaken connections on its own, a phenomenon called synaptic plasticity. The stimulation patterns used in TMS protocols closely mimic the electrical patterns that trigger long-lasting changes in synaptic strength in animal studies. This is likely why the benefits of a full course of TMS can persist for weeks or months after treatment ends, not just while the coil is active.
How a Treatment Session Works
Before your first session, a technician calibrates the machine to your brain. They place the coil over the part of your scalp that controls hand movement and fire test pulses at decreasing intensities until they find the lowest power that still makes your index finger twitch about half the time. This is your “motor threshold,” and it serves as a personalized baseline. The technician finds the right spot by moving the coil in half-centimeter increments, starting from roughly 4 centimeters to the side of the top of your head. Treatment intensity is then set as a percentage of this threshold.
Once calibrated, the coil is repositioned over the left prefrontal cortex, a region involved in mood regulation that tends to be underactive in people with depression. A standard course of TMS involves sessions five days a week for four to six weeks, typically 30 to 40 sessions total. Each session lasts anywhere from about 20 to 40 minutes depending on the protocol. During treatment, you’ll hear a loud clicking sound with each pulse and feel a tapping sensation on your scalp. Most people read, watch TV, or simply sit quietly.
A newer protocol called theta burst stimulation compresses the treatment into roughly 3 to 10 minutes per session by delivering pulses in rapid bursts that more closely mimic natural brain rhythms. Clinical evidence suggests it produces comparable results in a fraction of the time.
Different Coil Designs
The two main coil types you’ll encounter are the figure-8 coil and the H-coil. The figure-8 coil is the most widely used. It creates a focused stimulation area and reaches about 1.5 to 2 centimeters into the cortex. This precision is useful for targeting a specific brain region while minimizing stimulation of surrounding tissue.
The H-coil, used in devices marketed as “deep TMS,” generates a broader magnetic field that can reach structures roughly 2.5 to 3 centimeters deep. The trade-off is less precision: it stimulates a wider area of the brain. Both designs are FDA-cleared, and neither requires any incision or implant.
What TMS Is Approved to Treat
The FDA has cleared TMS primarily for major depressive disorder in patients who haven’t improved with antidepressant medication. This includes both adults and adolescents aged 15 to 21. TMS has also received FDA clearance for obsessive-compulsive disorder and for smoking cessation, and some devices are cleared for anxious depression and certain types of migraine.
Insurance coverage, including Medicare, generally requires documented evidence that previous medication trials have failed. Your provider will need to track your progress using standardized depression rating scales throughout the treatment course. Coverage criteria vary by insurer, but the consistent requirement is that TMS is positioned as a next step after medications haven’t worked, not a first-line treatment.
How Well It Works for Depression
For people with treatment-resistant depression (meaning at least two medications have failed), a meta-analysis of 19 randomized controlled trials found that about 40% of patients receiving real TMS had a meaningful clinical response, compared to roughly 14% of those receiving a sham (placebo) treatment. Even more striking, about 36% of TMS patients achieved full remission of their depression, compared to just 8% in the sham group. Patients receiving active TMS were nearly three times more likely to reach remission than those getting the placebo version.
These numbers are significant for a population that, by definition, has already failed to respond to standard treatments. TMS doesn’t work for everyone, but for a substantial portion of people who feel stuck, it offers a real chance of improvement without the systemic side effects of adding another medication.
Side Effects and Safety
The most common side effects are mild: scalp discomfort at the stimulation site and headache during or after sessions. Both tend to decrease over the course of treatment as you get used to the sensation.
The most serious potential risk is a seizure, but this is exceptionally rare. When TMS is delivered within established safety guidelines to people without additional risk factors, the seizure rate is less than one per 60,000 sessions. Even in broader estimates that include all coil types and protocols, the overall risk stays well below 1%. For context, the seizure risk associated with many commonly prescribed antidepressants is in a similar range. People with a history of seizures, metallic implants near the head, or certain neurological conditions are generally screened out before treatment begins.
TMS does not cause the memory loss or cognitive side effects associated with electroconvulsive therapy (ECT), which is one reason it has become an attractive alternative. There is no sedation, no recovery period, and no need for someone to drive you home.