Electrotherapy, including TENS and EMS devices, is generally safe for most adults. A large systematic review covering 381 studies found no serious adverse events directly attributable to TENS. The side effects that do occur are mild: skin irritation, redness, and occasional soreness at the electrode site. That said, certain medical conditions and specific body placements carry real risks, so the safety picture depends on who you are and how you use the device.
What the Side Effects Actually Look Like
The most common issue is skin irritation. Roughly 40% of people who use TENS regularly develop some form of contact dermatitis under the electrodes. This is usually caused by the adhesive in self-stick electrode pads, the propylene glycol in conductive gel, or the rubber and nickel in the electrodes themselves. For most people, it shows up as redness and itching that clears up after removing the pads.
Other reported side effects include mild nausea, discomfort from the stimulation sensation, temporary soreness at the electrode site, and occasional anxiety during use. All of these are considered manageable, and none required emergency intervention in clinical trials. If you find the adhesive pads irritating, switching to a hypoallergenic electrode brand or using a barrier spray on your skin before application can help.
Who Should Avoid Electrotherapy
The most serious safety concern involves people with implanted cardiac devices. Electrical stimulation can cause electromagnetic interference with pacemakers and implantable cardioverter defibrillators (ICDs). That interference can lead to device malfunction, either failing to detect a dangerous heart rhythm or misinterpreting the electrical signal as a rhythm that needs a shock. A systematic review in the Journal of Rehabilitation and Assistive Technologies Engineering concluded that TENS should not be used by anyone with an ICD, and that people with synchronous pacemakers should also avoid it. The only exception may be asynchronous pacemakers, though even then, caution is warranted.
Other groups who should avoid electrotherapy or use it only under professional guidance include people with epilepsy or a seizure history, since electrical current exposure can potentially trigger seizures, and people with active cancer, since electrodes should not be placed over or near cancerous lesions.
Where You Should Never Place Electrodes
Placement matters as much as the device itself. The FDA issues specific warnings about locations that carry serious physiological risks:
- The neck near the carotid sinus. Stimulation here can trigger a dangerous reflex that drops your heart rate and blood pressure, especially if you’re already sensitive to carotid sinus pressure.
- The front of the throat or mouth. Electrical current can cause severe spasms in the muscles that control your airway, potentially making it difficult to breathe.
- Across the chest (one electrode on each side). Sending current through the chest wall risks introducing electrical signals to the heart, which can cause irregular heart rhythms.
- Across the head or over the eyes. Stimulation applied across the brain is contraindicated for consumer devices.
- Over swollen, infected, or inflamed skin. This includes areas with phlebitis, thrombophlebitis, or varicose veins, where stimulation can worsen the underlying condition.
Safety During Pregnancy
Device manufacturers typically warn against using TENS during pregnancy, but clinical research tells a more nuanced story. TENS has been studied in all three trimesters, and the available evidence shows no differences in maternal or neonatal outcomes when basic precautions are followed. Some studies have even found that TENS can improve placental blood flow without harming the fetus.
The key rules for pregnant users: never place electrodes on the abdomen over the uterus, and avoid acupressure points associated with labor induction. These include the webbing between the thumb and index finger, several points on the lower leg and ankle, and the middle of the trapezius muscle on the shoulder. For back and pelvic pain, which is the most common reason pregnant people use TENS, electrodes placed on either side of the spine at the lower back and near the sacroiliac joints are considered appropriate. If uterine contractions develop during a session, stop immediately.
Using Electrotherapy With Metal Implants
If you have screws, plates, or a joint replacement, the safety picture depends on which type of electrotherapy you’re using. TENS and high-voltage stimulation have both been reported safe for people with orthopedic hardware, because neither delivers a sustained direct current to the tissue. The concern arises with galvanic (direct current) stimulation, which could theoretically cause localized heating where the current concentrates around metal. However, the guidance on this topic is largely opinion-based rather than backed by controlled experiments, so if you have metal implants and want to use electrotherapy near them, it’s worth discussing with your physical therapist.
Home Devices vs. Clinical Treatment
Both TENS and EMS devices sold directly to consumers are classified as Class II medical devices by the FDA, the same regulatory tier as powered wheelchairs and pregnancy tests. That classification requires manufacturers to meet performance standards covering electrical safety, electromagnetic compatibility, thermal safety, biocompatibility of the electrode materials, and software reliability. Devices marketed specifically for headache treatment face additional requirements, including clinical performance data.
The safety profile for home use appears comparable to clinical use. Studies that tracked adverse events in unsupervised home settings found the same mild issues seen in clinic: skin irritation, mild discomfort, and occasional redness. No serious adverse events emerged. The main risk with home use is placement error, putting electrodes in a location that could cause harm, or setting the intensity too high and causing skin burns. Reading the device manual and following the placement guidelines above will prevent most problems.
Long-Term Use
Many people use TENS daily for chronic pain, which raises reasonable questions about cumulative effects. The evidence is reassuring but incomplete. Across hundreds of studies, long-term use has not been linked to nerve damage, tissue injury, or any serious adverse event. The only consistent finding is that some people develop skin sensitivity to electrode adhesives over time, which is the contact dermatitis issue mentioned earlier. Rotating electrode placement sites and giving your skin breaks between sessions can minimize this.
One gap in the research is that few studies have rigorously tracked outcomes beyond a few months of continuous use. Data at 6 months and 12 months after starting treatment is scarce. What does exist, though, shows no safety signals that would discourage ongoing use. The practical limit for most people is skin tolerance rather than any deeper physiological concern.