The idea that legal cannabis can help address the opioid crisis has generated much hope and enthusiasm.
Opioid misuse has declined in recent years at the same time that cannabis use has been increasing, with many states liberalizing marijuana laws.
Based on recent research, some advocates have been promoting this connection, arguing that easier access to marijuana reduces opioid use and, in turn, overdose deaths.
A new study urges caution. Sometimes appearances — or statistics — can be deceiving.
Why people were so hopeful
It’s plausible that marijuana can help reduce pain. Systematic reviews show that certain compounds found in marijuana or synthetically produced cannabinoids do so, at least for some conditions. So some people who might otherwise seek out opioid painkillers could use medical marijuana instead.
Regulations in some states, including New York, that streamline access to medical marijuana are based on the idea that it can substitute for opioids in pain treatment.
In 2014, a study published in JAMA Internal Medicine gave further hope that liberalizing marijuana laws might alleviate the opioid crisis.
The study examined the years 1999 through 2010, during which 10 states established medical marijuana programs. It compared changes in the rates of opioid painkiller deaths in states that passed medical marijuana laws with those that had not. The results? Researchers found that the laws were associated with a nearly 25 percent decline in the death rate from opioid painkillers.
Since publication of the JAMA study, others have produced similar findings. One posted last fall at the Social Science Research Network found that counties with medical marijuana dispensaries have up to 8 percent fewer opioid-related deaths among non-Hispanic white men, and 10 percent fewer heroin deaths.
Other studies have documented marijuana laws associated with reduced opioid prescribing in Medicaid and Medicare.
Why you should be skeptical
None of this proves that marijuana liberalization causes lower opioid-related mortality, something the authors of the 2014 JAMA study pointed out.
Correlation does not mean causation, of course. A particular challenge in interpreting correlations in social science has its own name — the ecological fallacy. It’s the erroneous conclusion that relationships observed at the wider level (like state or region) necessarily hold true at the individual level as well.
“It’s possible that relationships get strengthened, weakened or even reversed when going from the individual to aggregate level,” said Mark Glickman, senior lecturer on statistics at Harvard. This was documented in a classic paper in 1950 and underlies many erroneous conclusions from research.
A new study revisited the JAMA-published analysis with more data. Its conclusions cast doubt on the idea that medical marijuana helps reduce opioid deaths — at least as far as we can tell with state-level data.
Between 2010 — the final year of analysis in the JAMA study — and 2017, 32 more states legalized medical marijuana, and eight legalized recreational use. A new study published in the Proceedings of the National Academy of Sciences (P.N.A.S.) reassessed the relationship between these laws and opioid deaths using the same approach as the JAMA study, but extending the years of analysis through 2017.
Over the years analyzed in the JAMA study, 1999 to 2010, the new P.N.A.S. study produced similar findings: Medical marijuana legalization was associated with reduced opioid painkiller overdose deaths. But in an expanded analysis through 2017, the results reversed — the laws are associated with a 23 percent increase in deaths.
This doesn’t necessarily mean that the laws first saved lives and then, in later years, contributed to deadly overdoses.
“If there is a relationship between medical cannabis use and opioid overdose on an individual level, this kind of study can’t reveal it,” said the epidemiologist Chelsea L. Shover, lead author of the P.N.A.S. study and a postdoctoral research fellow at Stanford School of Medicine.
Mr. Glickman said: “Other sources of potential bias are possible in all these studies. As states were liberalizing their marijuana laws, they could have been changing other policies that also affected opioid overdoses.” We could be misled by analyses that don’t also control for those.
For example, efforts to expand treatment for opioid abuse could reduce deaths. Or, imprisoning more opioid users could increase it, because studies have found that overdoses are likelier after a prison stay.
Other evidence suggests it’s unlikely that easier access to marijuana could significantly reduce opioid painkiller death rates. For one thing, medical marijuana users make up only about 2.5 percent of the adult population, a small proportion of the 38 percent that receive a prescription for an opioid painkiller each year.
For another, some work casts doubt on the idea that medical marijuana substitutes for opioids. One study found that medical marijuana use is positively correlated both with use and misuse of prescription medications, including pain relievers.
Another found that patients on opioid painkillers who also use medical marijuana have higher doses of opioids than those who don’t use medical marijuana. And another study found that marijuana use tends to increase the chances of developing an opioid use disorder.
“Given what we know from research, we should decouple our thinking about medical cannabis and opioid overdoses,” Ms. Shover said. “However, there are other ways to address the opioid crisis.”
These include increasing availability of the overdose reversal drug naloxone and improving access to medication-assisted treatment.
Easier and legal ways to obtain medical (and even recreational) marijuana may have many benefits. But enthusiasm should be tempered that reducing opioid deaths is one of them.
