What is TENS? Transcutaneous Electrical Nerve Stimulation is a safe, non-invasive, drug free method of controlling pain. It reduces pain by sending small electrical impulses through electrodes that are placed on the skin to underlying nerve fibers to modify your pain perception. Two popular theories help explain how TENS works: 1) The Gate Control Theory suggests that the electrical stimulation of the nerve fibers close a segment of the spinal cord, which blocks the pain signals from reaching the brain. 2) The Endorphin Release Theory suggests that TENS stimulation reduces inflammation, swelling and relaxes muscle fibers by releasing endorphins in the brain which act like analgesics. The TENS units for sale here provide a safe, non-addictive method of electrotherapy pain relief which has virtually no side effects.