Transcutaneous electrical nerve stimulation (TENS or TNS) is the use of electric current produced by a device to stimulate the nerves for therapeutic purposes. TENS, by definition, covers the complete range of transcutaneously applied currents used for nerve excitation although the term is often used with a more restrictive intent, namely to describe the kind of pulses produced by portable stimulators used to reduce pain. The unit is usually connected to the skin using two or more electrodes which are typically conductive gel pads. A typical battery-operated TENS unit is able to modulate pulse width, frequency and intensity. Generally TENS is applied at high frequency (>50 Hz) with an intensity below motor contraction (sensory intensity) or low frequency (<10 Hz) with an intensity that produces motor contraction. While the use of TENS has proved effective in clinical studies, there is controversy over which conditions the device should be used to treat.
TENS devices are available without a prescription and are used as a non-invasive nerve stimulation intended to reduce both acute and chronic pain. One review from 2007 felt that the evidence supports a benefit in chronic musculoskeletal pain. Results from a task force on neck pain in 2008 found no clinically significant benefit to TENS for the treatment of neck pain when compared to a placebo treatment. A 2010 review did not find evidence to support the use of TENS for chronic low back pain. There is tentative evidence that it may be useful for painful diabetic neuropathy. As of 2015, the efficacy of TENS therapy for phantom limb pain is not known as no randomized controlled trials have been performed.
In principle, an adequate intensity of stimulation is necessary to achieve pain relief with TENS. An analysis of treatment fidelity (meaning that the delivery of TENS in a trial was in accordance with current clinical advice, such as using "a strong but comfortable sensation" and suitable, frequent treatment durations) showed that higher fidelity trials tended to have a positive outcome.
A few studies have shown objective evidence that TENS may modulate or suppress pain signals in the brain. One used evoked cortical potentials to show that electric stimulation of peripheral A-beta sensory fibers reliably suppressed A-delta fiber nociceptive processing. Two other studies used functional magnetic resonance imaging (fMRI): one showed that high-frequency TENS produced a decrease in pain-related cortical activations in patients with carpal tunnel syndrome, while the other showed that low-frequency TENS decreased shoulder impingement pain and modulated pain-induced activation in the brain.
A head-mounted TENS device called Cefaly was approved by the United States Food and Drug Administration, in March 2014, for the prevention of migraines. The Cefaly device was found effective in preventing migraine attacks in a randomized sham-controlled trial. This was the first TENS device the FDA approved for pain prevention, as opposed to pain suppression.
A study performed on healthy human subjects demonstrates that repeated application of TENS can create analgesic tolerance within five days, reducing its efficacy. The study noted that TENS causes the release of endogenous opioids, and that the analgesia is likely due to opioid tolerance mechanisms. 
Earlier studies have stated that TENS "has been shown not to be effective in postoperative and labour pain." These studies also had questionable ability to truly blind the patients.  However, more recent studies have shown that TENS was "effective for relieving labour pain, and they are well considered by pregnant participants." One study also showed that there was a significant change in how soon the laboring people took to request pharmacologic pain management, like the epidural. The group with the TENS waited five additional hours. Both groups were satisfied with the pain relief that they had from their choices. No maternal, infant, or labor problems were noted. 
Electrical stimulation for pain control was used in ancient Rome, 63 A.D. It was reported by Scribonius Largus that pain was relieved by standing on an electrical fish at the seashore. In the 16th through the 18th century various electrostatic devices were used for headache and other pains. Benjamin Franklin was a proponent of this method for pain relief. In the 19th century a device called the electreat, along with numerous other devices were used for pain control and cancer cures. Only the electreat survived into the 20th century, but was not portable, and had limited control of the stimulus. Development of the modern TENS unit is generally cred to C. Norman Shealy.
The first modern, patient-wearable TENS was patented in the United States in 1974. It was initially used for testing the tolerance of chronic pain patients to electrical stimulation before implantation of electrodes in the spinal cord dorsal column. The electrodes were attached to an implanted receiver, which received its power from an antenna worn on the surface of the skin. Although intended only for testing tolerance to electrical stimulation, many of the patients said they received so much relief from the TENS itself that they never returned for the implant.
A number of companies began manufacturing TENS units after the commercial success of the Medtronic device became known. The neurological division of Medtronic, founded by Don Maurer, Ed Schuck and Charles Ray, developed a number of applications for implanted electrical stimulation devices for treatment of epilepsy, Parkinson's disease, and other disorders of the nervous system.
Today many people confuse TENS with electrical muscle stimulation (EMS). EMS and TENS devices look similar, with both using long electric lead wires and electrodes. TENS is for blocking pain, where EMS is for stimulating muscles.
TENS used across an artificial cardiac pacemaker (or other indwelling stimulator, including across its leads) may cause interference and failure of the implanted device. Serious accidents have been recorded in cases when this principle was not observed. A 2009 review in this area suggests that electrotherapy, including TENS, is "best avoided" in patients with pacemakers or implantable cardioverter-defibrillators (ICDs). They add that "there is no consensus and it may be possible to safely deliver these modalities in a proper setting with device and patient monitoring", and recommend further research. The review found several reports of ICDs administering inappropriate treatment due to interference with TENS devices, but notes that the reports on pacemakers are mixed: some non-programmable pacemakers were inhibited by TENS, but others were unaffected or auto-reprogrammed.
The use of TENS is likely to be less effective on areas of numb skin or decreased sensation due to nerve damage. It may also cause skin irritation due to the inability to feel currents until they are too high. There's an unknown level of risk when placing electrodes over an infection (possible spreading due to muscle contractions), but cross contamination with the electrodes themselves is of greater concern. TENS should also be used with caution in people with epilepsy or pregnant women; do not use over area of the uterus as the effects of electrical stimulation over the developing fetus are not known.
^Johnson M, Martinson M (2007). "Efficacy of electrical nerve stimulation for chronic musculoskeletal pain: A meta-analysis of randomized controlled trials". Pain. 130 (1–2): 157–165. doi:10.1016/j.pain.2007.02.007. PMID17383095.
^ abDubinsky RM, Miyasaki J (2009). "Assessment: Efficacy of transcutaneous electric nerve stimulation in the treatment of pain in neurologic disorders (an evidence-based review): Report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology". Neurology. 74 (2): 173–176. doi:10.1212/WNL.0b013e3181c918fc. PMID20042705.
^Bjordal JM, Johnson MI, Ljunggreen AE (2003). "Transcutaneous electrical nerve stimulation (TENS) can reduce postoperative analgesic consumption. A meta-analysis with assessment of optimal treatment parameters for postoperative pain". European Journal of Pain. 7 (2): 181–188. doi:10.1016/S1090-3801(02)00098-8. PMID12600800.
^Bennett MI, Hughes N, Johnson MI (2011). "Methodological quality in randomised controlled trials of transcutaneous electric nerve stimulation for pain: Low fidelity may explain negative findings". Pain. 152 (6): 1226–1232. doi:10.1016/j.pain.2010.12.009. PMID21435786.
^Kara M, Ozçakar L, Gökçay D, Ozçelik E, Yörübulut M, Güneri S, Kaymak B, Akinci A, Cetin A (2010). "Quantification of the Effects of Transcutaneous Electrical Nerve Stimulation with Functional Magnetic Resonance Imaging: A Double-Blind Randomized Placebo-Controlled Study". Archives of Physical Medicine and Rehabilitation. 91 (8): 1160–1165. doi:10.1016/j.apmr.2010.04.023. PMID20684895.
^Kocyigit F, Akalin E, Gezer NS, Orbay O, Kocyigit A, Ada E (2012). "Functional Magnetic Resonance Imaging of the Effects of Low-frequency Transcutaneous Electrical Nerve Stimulation on Central Pain Modulation". The Clinical Journal of Pain. 28 (7): 581–588. doi:10.1097/AJP.0b013e31823c2bd7. PMID22699130.
^Schoenen J, Vandersmissen B, Jeangette S, Herroelen L, Vandenheede M, Gérard P, Magis D (Feb 2013). "Migraine prevention with a supraorbital transcutaneous stimulator: a randomized controlled trial". Neurology. 80 (8): 697–704. doi:10.1212/WNL.0b013e3182825055. PMID23390177.
^McQuay HJ, Moore RA, Eccleston C, Morley S, Williams AC (July 1997). "Systematic review of outpatient services for chronic pain control". Health Technology Assessment. 1 (6): i–iv, 1–135. doi:10.3310/hta1060. PMID9483161.
^van der Spank JT, Cambier DC, De Paepe HM, Danneels LA, Witvrouw EE, Beerens L (2000). "Pain relief in labour by transcutaneous electrical nerve stimulation (TENS)". Archives of Gynecology and Obstetrics. 264 (3): 131–136. doi:10.1007/s004040000099. PMID11129512.
^Santana, S (January 2016). "Transcutaneous electrical nerve stimulation (TENS) reduces pain and postpones the need for pharmacological analgesia during labour: a randomised trial". J Physiother. 62 (1): 29–34. doi:10.1016/j.jphys.2015.11.002. PMID26701166.
^ abcChipaila, Nicolae; Sgolastra, Fabrizio; Spadaro, Alessandro; Pietropaoli, Davide; Masci, Chiara; Cattaneo, Ruggero; Monaco, Annalisa (2014-04-01). "The effects of ULF-TENS stimulation on gnathology: the state of the art". Cranio: The Journal of Craniomandibular Practice. 32 (2): 118–130. doi:10.1179/0886963413Z.00000000018. ISSN0886-9634. PMID24839723.
^Jensen JE, Conn RR, Hazelrigg G, Hewett JE (1985). "The use of transcutaneous neural stimulation and isokinetic testing in arthroscopic knee surgery". The American Journal of Sports Medicine. 13 (1): 27–33. doi:10.1177/036354658501300105. PMID3872082.
Cekmen N, Salman B, Keles Z, Aslan M, Akcabay M (Feb 2007). "Transcutaneous electrical nerve stimulation in the prevention of postoperative nausea and vomiting after elective laparoscopic cholecystectomy". J Clin Anesth. 19 (1): 49–52. doi:10.1016/j.jclinane.2006.05.025. PMID17321927.
Gan LS, Prochazka A, Bornes TD, Denington AA, Chan KM (Mar 2007). "A new means of transcutaneous coupling for neural prostheses". IEEE Trans Biomed Eng. 54 (3): 509–17. doi:10.1109/TBME.2006.886664. PMID17355064.