Towards Rigorous Assessment of Marijuana-Opioid Substitution Effects

A number of correlational studies have shown that at the individual level, cannabis use is positively associated with opioid use, misuse, and addiction, whereas at the state level, availability is negatively associated with opioid prescriptions and overdose rates. Many people have responded to this ambiguous situation by asserting that the correlations which fit their marijuana politics represent rigorous causal evidence and those that don’t are just correlations, nothing to see here, let’s move on.

Assuming you actually wanted to know the answer about whether cannabis and opioids are substitutes, and saw science as a tool to learn things rather than prop up your ideology, what would be a rigorous way to test the question? Experimental designs are tough to implement because of the ethical issues surrounding introducing people to addictive drugs. However, my colleague Beth Darnall and I lay out a strategy in the current issue of Addiction that doesn’t present this ethical quandary.

Millions of people are taking prescribed opioids long-term and many of them want to reduce their dose or stop taking them entirely. Tapering protocols have been developed to help such people. Critically, a significant number of these people also use cannabis.

This presents an opportunity: Run a randomized clinical trial on such individuals who are interested in tapering opioids and also measure what happens to their cannabis use if their opioid use changes. Beth did this is a small tapering study which didn’t have the power to assess effects rigorously but demonstrated proof of concept (FWIW, three people quit marijuana use, all of whom had also quit opioid use). With support from PCORI she has now launched a much bigger study in which this question can be addressed within a multi-site randomized trial of opioid tapering.

This isn’t the only route to pushing the science in this area. Ziva Cooper at Columbia is doing inventive experiments on the interaction of opioid use, cannabis use, pain, and drug liking. It’s an intriguing area and good science will out, even if only a minority of people are willing to accept the results whatever they are.

Author: Keith Humphreys

Keith Humphreys is the Esther Ting Memorial Professor of Psychiatry at Stanford University and an Honorary Professor of Psychiatry at Kings College London. His research, teaching and writing have focused on addictive disorders, self-help organizations (e.g., breast cancer support groups, Alcoholics Anonymous), evaluation research methods, and public policy related to health care, mental illness, veterans, drugs, crime and correctional systems. Professor Humphreys' over 300 scholarly articles, monographs and books have been cited over thirteen thousand times by scientific colleagues. He is a regular contributor to Washington Post and has also written for the New York Times, Wall Street Journal, Washington Monthly, San Francisco Chronicle, The Guardian (UK), The Telegraph (UK), Times Higher Education (UK), Crossbow (UK) and other media outlets.