The length of time the pain killing effects last once you switch off the TENS. Some patients have reported it can last up to 24 hours, and that the results improve after multiple TENS sessions. Now and again, a few patients experience extremely transient (enduring just a couple of hours) reactions, for example, feeling tired, drained, spacey, or have a gentle migraine. Think about these reactions as a positive reaction, as a sign that the patient’s mind is associating with the LENS. This is normally a decent prognostic marker.