Different pain reducing drugs work in different ways to reduce pain. Many drugs, including morphine and codeine, reduce pain by binding to opiate receptors in the brain. Another chemical, naloxone, can block the effect of these drugs. In this study, we are looking at brain responses to a pain-relieving cream and whether naloxone blocks the effect of the cream. We will compare the results of people with chronic pain (fibromyalgia) to those of people without chronic pain.