Transcutaneous Electrical Nerve Stimulation (TENS) is predominantly used for the treatment of nerve related pain conditions. A TENS machine is generally considered safe, although it is not used in individuals who have a pacemaker or who are in the first weeks of pregnancy. By definition, the unit applies electrical stimulation transcutaneously across a broad range of anatomy. Electrical stimulation was first used in ancient Rome to relieve pain by using electrical fish from the sea. Later, a variety of other devices were used to treat headachesand other pain. There are several anatomical locations where the use of the units is contraindicated.  A TENS unit should not be used over the eyes, through the chest, on broken skin, over a tumor, directly over the spinal cord, or internally.