- Researchers want to look at how the brain responds to painful stimulations. They also want to see if these responses are different in people with and without chronic pain. To test the brain s response, they will use a chemical called naloxone. Naloxone is used to treat overdoses of painkilling drugs like morphine. It may be able to block the effect of a pain-relieving cream. Researchers will apply a pain-relieving cream to a person s lower leg and look at the results of sensitivity tests with either naloxone or a placebo. This study will compare the results from people with chronic pain (like fibromyalgia) to those of people without chronic pain.