Despite being illegal under US federal law, cannabis has gained widespread public acceptance. Over half of US adults (59%) believe that cannabis should be legal for recreational and medical purposes (1), and nearly one in five (19.6%) report using cannabis in the past year (2). Cannabis use is particularly common among individuals with opioid use disorder. In clinical samples of people with opioid use disorder, approximately half (51%)(3) endorse past-month cannabis use and one-quarter (28%)(4) meet criteria for cannabis use disorder.

As the United States confronts an unprecedented crisis of opioid-related morbidity and mortality, some advocates have asserted that cannabis should be part of the solution. Advertisements from the for-profit marijuana industry have appealed to young people and have implied that cannabis can treat opioid addiction (5). Several states permit authorizing medical cannabis for opioid addiction. Although there have been no randomized controlled trials demonstrating that cannabis benefits people with opioid use disorder, this controversial concept has impacted drug policy, clinical practice, and public perceptions of viable alternatives for individuals struggling with opioid addiction. Prospective cohort studies offer a potentially informative window into the possibility that cannabis can replace opioids in adults with opioid use disorder. If, among people with opioid use disorder, an increase in cannabis use is associated with a decline in opioid use, it would support the rationale for fielding clinical trials to evaluate whether cannabis has a therapeutic role in opioid addiction. Prior research on this key point has offered mixed results. Two previous cohort studies have provided qualified support for an inverse association between cannabis and opioid use among people who use drugs. First, a group-based trajectory study of patients receiving medications for opioid use disorder (MOUD) revealed that patients in the rapidly decreasing and continued low-level opioid use groups had an increased likelihood of also being in the high and increasing cannabis use group (6). Within these two opioid groups, however, increasing cannabis use was not associated with completion of MOUD treatment as would be expected if cannabis use were conferring clinical benefit. Second, a longitudinal study of middle-aged adults with persistent pain and illicit drug use found that compared with cannabis non-users, those who used cannabis every day were about