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.