PROJECT SUMMARY / ABSTRACT Controversy exists over the risk-to-benefit ratio of medical marijuana (MM) for adults with chronic non-cancer pain (CNCP) on chronic opioid therapy (COT). Approximately 50 million adults in the United States suffer from CNCP, a debilitating medical condition that is complex to manage. The majority of those with CNCP are treated with COT, but the evidence supporting long-term effectiveness of opioid drugs for pain and improved functional status is weak, and high-dose COT greatly increases risk for opioid use disorder (OUD) and opioid overdose death. Because of the complex medical needs of this patient group, there has been enthusiasm over the potential to treat CNCP with MM. However, evidence to support the effectiveness of MM to either treat chronic pain, or to reduce opioid use, is weak, and there are risks associated with MM. Thus, the issue of whether CNCP patients using opioids should use MM as an adjuvant therapy to COT remains controversial, and there have been no randomized studies that have directly addressed this question. We propose to enroll 250 adults who endorse >6 months of CNCP with neuropathic pain, have received COT at a dose of 50-300 MME/day for >90 days, who have not used but are considering use of MM, and who report interest in tapering their COT dose, into a randomized, pragmatic 3 site trial. Participants will be randomly assigned to begin MM at enrollment or to a waitlist control condition (WL), and not initiate MM for 24 weeks. All participants will be offered a 24-week, manualized behavioral prescription opioid taper support (POTS) program. We will evaluate whether patients with CNCP on COT who are assigned to MM+POTS, compared with those assigned to WL+POTS, show (1) greater COT dose reduction (MME/day), and/or greater improvement in pain severity on a pain numeric rating scale (co-primary outcomes), (2) improved quality of life, (3) improved pain interference, depression, and anxiety; and (4) improvement in cognitive functioning (working memory, attention, and executive function performance), from baseline to 24 weeks. We will also assess the development of CUD in those using MM. Urine collected at each visit will be assessed quantitatively for cannabis metabolites. The proposed study will answer a timely and significant public health question regarding whether MM use with behavioral support, compared to behavioral support alone, is beneficial or harmful for this population. This information is critical for patients, healthcare providers, and policymakers to evaluate the risks and benefits of MM as an adjuvant treatment to COT for CNCP.