Treatment of chronic pain typically relies on the effectiveness of opioid analgesics, however, side effects and the potential for users to develop tolerance and addiction may make physicians hesitant to prescribe them. The development of opioid analgesics with fewer side effects and decreased potential tolerance and addiction would increase the ability to safely and effectively treat pain. This study will determine whether the addition of an ultra-low dose of naloxone, an opioid antagonist, to buprenorphine, an opioid analgesic pain medication, increases the analgesic potency and reduces side effects compared to buprenorphine alone. Low doses of naloxone added to buprenorphine have been shown to increase the potency and reduce the side effects of buprenorphine in animal studies and in a human study with healthy volunteers. The current study aims to extend these findings by assessing the analgesic potency and side effects of a 15:1 ratio of buprenorphine to naloxone in a sample of 12 outpatient participants with moderate pain (4 or greater pain intensity) who are experienced with opioid analgesics. Participants will be randomly assigned to begin blinded treatment starting with one of two medication regimens: IV buprenorphine + ultra-low-dose naloxone or IV buprenorphine alone. Participants receive 5 daily doses of medication (no more than 2 days between each dose) followed by crossover to the opposite treatment condition for 5 additional daily doses. Each study day, participants will provide a daily pain assessment, receive administration of study medication, and will then be monitored and assessed for pain and side effects over the next six hours. Outcomes for the study include: (1) pain intensity as measured using the Brief Pain Inventory, pain intensity numerical rating scale, and the Beck Depression Inventory, and (2) self-reported side effects/adverse events. After completion of study participation, participants will resume pre-study pain treatments with their own physician.