The following originally appeared on The Upshot (copyright 2019, The New York Times Company).
The idea that legal cannabis can help address the opioid crisis has generated much hope and enthusiasm.
Based on recent research, some advocates have been promotingthis 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.
It’s plausible that marijuana can help reduce pain. Systematicreviews 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.
In 2014, a study published in JAMA 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.
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.