Safety & Side Effects of TMS Part 3:Rare Adverse Effects and Mitigation Strategies
Transcranial Magnetic Stimulation (TMS) is known for having a very low risk of serious side effects, especially when compared to medications or more invasive procedures like electroconvulsive therapy (ECT). However, like with any medical treatment, rare adverse events can occur. Fortunately, these effects are extremely uncommon, and when clinics follow proper screening protocols, the risks can be reduced even further.
This section takes a closer look at the rare but potential risks of TMS—and the steps taken to prevent them.
Seizures: The Most Serious but Extremely Rare Risk
The most widely discussed rare side effect of TMS is the potential for triggering a seizure. While this sounds alarming, it’s important to understand just how uncommon it is. According to safety guidelines from the international TMS community, the risk of seizure during a TMS session is estimated to be less than 1 in 30,000 treatments when proper screening and protocols are followed.
To put that in perspective, your chance of having a seizure from playing a video game with flashing lights may actually be higher if you have a seizure disorder. Still, TMS clinics take this risk very seriously. That’s why anyone with a history of epilepsy or seizures is usually excluded from treatment, even if the condition has been inactive for years.
Additional factors that may increase seizure risk include:
- Taking certain medications that lower seizure threshold
- Severe sleep deprivation
- Substance abuse or withdrawal
Substance Use and Seizure Risk
Substance use—especially alcohol, stimulants (like cocaine or methamphetamine), and certain prescription drugs—can significantly increase the brain’s sensitivity and excitability. When a person is actively using or withdrawing from these substances, the brain’s electrical system is already under stress. TMS, which works by stimulating the brain with magnetic pulses, adds another layer of stimulation. In rare cases, this combination may push the brain’s excitability past a safe threshold, raising the risk of a seizure.
Even substances that are not currently in the system have caused long-term neurological effects (like chronic alcohol abuse or benzodiazepine dependency) can make the brain more vulnerable. That’s why a full history of drug and alcohol use is part of the mandatory pre-treatment screening process. If there’s any concern about recent or unstable use, clinics will either delay treatment until the patient is stable or work in partnership with addiction specialists to create a safer care plan.Clinics carefully review all of this during the pre-treatment evaluation to ensure a patient’s safety. In real-world practice, seizures from TMS are exceedingly rare and are almost always linked to known risk factors that can be screened out.
Mania or Hypomania in Bipolar Disorder
Although TMS is generally safe for individuals with bipolar depression, there have been rare cases of hypomanic or manic episodes triggered during treatment—especially in patients who have had unstable moods in the past. This is why bipolar patients are often closely monitored and may require modified stimulation protocols or mood-stabilizing medications alongside TMS.
The chance of mania happening is still low, but the key is proper diagnosis and provider awareness. Patients should be encouraged to share any past mood episodes—even if they didn’t require hospitalization—so the treatment team can build a plan that minimizes risk.
Worsening Anxiety or Agitation
In rare instances, TMS may lead to temporary increases in anxiety, restlessness, or emotional instability. This may occur as part of the brain’s early response to stimulation, especially in patients with anxiety or trauma-related conditions.
This type of response is usually short-lived and may be handled by reducing the frequency of sessions, lowering stimulation intensity, and/or adding relaxation strategies or therapy alongside TMS. For most patients, these symptoms settle within the first 1–2 weeks. In rare cases where symptoms persist, clinicians may pause treatment and re-evaluate the protocol.
Syncope (Fainting) or Cardiovascular Reactions
Although very uncommon, brief fainting episodes (known as vasovagal syncope) have been reported during or immediately after TMS sessions. These are often triggered by anxiety, dehydration, or skipping meals—not the stimulation itself. These events are not seizures and do not cause long-term harm.
To reduce this risk, providers advise patients to eat a light meal beforehand, stay hydrated, and let staff know if they’re feeling lightheaded. Patients are always seated during treatment and are observed throughout, so if fainting does occur, it is managed quickly and safely.
How Clinics Keep TMS Safe
To prevent rare side effects, TMS providers take multiple steps before treatment even begins:
- Detailed medical history review to screen for seizure risk, bipolar disorder, and medication interactions
- Standardized protocols that follow international safety guidelines
- Real-time patient feedback to make ongoing adjustments based on how the patient feels during treatment
- Emergency protocols in the rare event of a serious reaction
These steps have made TMS one of the most well-monitored and controlled therapies in modern psychiatry.
Final Takeaway
While rare side effects like seizures or mood shifts can happen, they are extremely uncommon and largely preventable. With proper screening, supervision, and clinical judgment, TMS remains one of the safest treatment options available—especially for people who haven’t responded to medications.
Being aware of these rare possibilities doesn’t mean you should be afraid of treatment—it just means you and your provider are working together to make your experience as safe and effective as possible.
Sources
- Rossi, S., Antal, A., Bestmann, S., et al. (2021). Safety and recommendations for TMS use in clinical practice and research. Clinical Neurophysiology, 132(1), 269–306. https://doi.org/10.1016/j.clinph.2020.10.003
- Perera, T., George, M. S., Grammer, G., et al. (2016). The Clinical TMS Society consensus review and treatment recommendations for TMS therapy for major depressive disorder. Brain Stimulation, 9(3), 336–346. https://doi.org/10.1016/j.brs.2016.03.010
- Holtzheimer PE 3rd, McDonald WM, Mufti M, Kelley ME, Quinn S, Corso G, Epstein CM. Accelerated repetitive transcranial magnetic stimulation for treatment-resistant depression. Depress Anxiety. 2010 Oct;27(10). https://pubmed.ncbi.nlm.nih.gov/20734360/
- Wassermann, E. M. (1998). Risk and safety of repetitive TMS: Report and suggested guidelines. Electroencephalography and Clinical Neurophysiology, 108(1), 1–16. https://doi.org/10.1016/S0168-5597(97)00096-8
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