TENS units should only be used under the direction of a doctor, physical therapist, or occupational therapist. Electrodes are attached to the surface of the skin over or near the area where you are experiencing pain. It is important that you learn how to properly place the electrodes; operate the unit; change the batteries; vary the controls and settings (frequency and voltage); set the proper duration and intensity of the stimulation (depending on the location and type of pain).
TENS units are prescribed for both acute pain and chronic pain conditions. For some chronic pain patients, a TENS unit provides pain relief that can last for several hours. For others, a TENS unit may help reduce the amount of pain medications needed. Some patients hook the unit onto a belt, making it more portable, so they can turn it on and off as needed.
The cost of a TENS unit can range from about $100 to several hundred dollars. TENS units can be purchased or rented. A prescription usually is necessary for insurance reimbursement of a TENS unit.
Transcutaneous electrical nerve stimulators TENS, Spine-health.com