Naloxone is a mu-opioid-receptor (MOR) antagonist drug, which dose-dependently exhibits hypo-algesic and hyper-algesic properties. Naloxone (and other MOR-antagonists) have been used in research to study the role of endogenous opioids in central processing of pain. It has been hypothesized that the endogenous opioid modulation of pain is impaired or altered in chronic pain conditions.