Five years ago, a study of death certificate data attracted notice for suggesting that states that passed medical marijuana laws saw 25 percent fewer opioid overdose deaths on average than states that barred medical cannabis.
The authors were careful to point out that this finding was only a correlation, an intriguing hint at something that needed further exploration. There was no way to establish whether the availability of medical cannabis in some states protected against overdosing on harder drugs, even if some people used marijuana for pain.
Nevertheless, the cannabis industry took up the study to help win passage of medical cannabis laws in more states, even as medical experts expressed skepticism. In a 2018 report, for example, Maryland’s medical marijuana commission found “no credible scientific evidence” that marijuana could treat opioid addiction.