In principle, an adequate intensity of stimulation is necessary to achieve pain relief with TENS. Potential benefits of TENS treatment include its safety profile, relative affordability, ease of self-administration, and availability over-the-counter without a prescription. Many systematic reviews and meta-analyses assessing clinical trials looking at the efficacy of TENS for different sources of pain, however, have been inconclusive due to a lack of high-quality and unbiased evidence. Transcutaneous electrical nerve stimulation is a commonly used treatment approach to alleviate acute and chronic pain by reducing the sensitization of dorsal horn neurons, elevating levels of gamma-aminobutyric acid and glycine, and inhibiting glial activation. While the use of TENS has proved effective in clinical studies, there is controversy over which conditions the device should be used to treat. Intensity of stimulation should be strong but comfortable with greater intensities, regardless of frequency, producing the greatest analgesia. More recently, many TENS units use a mixed frequency mode which alleviates tolerance to repeated use. 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. A typical battery-operated TENS unit is able to modulate pulse width, frequency, and intensity. The unit is usually connected to the skin using two or more electrodes which are typically conductive gel pads. TENS, by definition, covers the complete range of transcutaneously applied currents used for nerve excitation, but 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. A transcutaneous electrical nerve stimulation ( TENS or TNS) is a device that produces mild electric current to stimulate the nerves for therapeutic purposes.