Transcutaneous Electrical Nerve Stimulation (TENS) is the most commonly used electrical stimulation device to apply electrotherapy to the body for the treatment of back pain. A TENS unit has electrodes that can be placed over the painful area, over the nerve supplying the painful area or on the opposite side of the body. The user can adjust the intensity of electrical stimulation and chose either high or low frequency. The exact mechanism of electrical stimulation's beneficial effect is not known, although it is thought that TENS may effectively treat pain by directly blocking transmission of pain signals along nerves. In addition, electrical stimulation has been shown to promote the release of endorphins, which are natural painkillers produced by the body.