This is a 2016 study that was a collaboration between the Washington University School of Medicine, The Hadassah-Hebrew University Medical Center published in the Clinical Journal of Pain.
Findings included: “the treatment of chronic pain with medicinal cannabis…resulted in improved pain and functional outcomes and significant reduction in opioid use.
They also found that “opioid consumption at follow up decreased by 44% (P<0.001)
Clinical Journal of Pain
Haroutounian, Ratz, Ginosar, Furmanov, Sanifi, Meidan, Davidson. The Effect of Medicinal Cannabis on Pain and Quality-of-Life Outcomes in Chronic Pain: A Prospective Open-label Study. Clinical Journal of Pain 2016; 32(12) 1036-1043