Transcutaneous Electrical Nerve Stimulation (TENS) is typically used in the treatment of nerve related pain conditions. This form of electrical stimulation was first used in ancient Rome, using electrical fish from the sea, in order to relieve pain. The machines now are considered safe but should not be used on individuals with pacemakers or in the first few weeks of pregnancy. TENS machines work by applying electrical stimulation to a broad range of anatomy.
Although it is primarily safe, there are several anatomical locations where the unit is contraindicated: over the eyes, through the chest, over a tumor, on broken skin, internally, or directly over the spinal cord.