In the 2014 study, published in JAMA Internal Medicine, researchers from Johns Hopkins Bloomberg School of Public Health, Montefiore Medical Center and elsewhere looked at the period 1999 to 2010. They found 24.8 percent fewer deaths after states passed laws legalizing medical marijuana.
The Stanford research team replicated the earlier work in a study published in the Proceedings of the National Academy of Sciences. For the period from 1999 to 2010, they came up with very similar results — a 21.1 percent reduction in opioid overdose deaths. But once they extended the analysis to 2017, and even when they controlled for new laws allowing recreational use of marijuana and other factors, the data reversed.
Two of the researchers in the initial study did not return emails Monday seeking comment.
Shover noted that for the initial study, only 13 states had medical marijuana laws and many of them were in the west, where the opioid epidemic arrived later. That may have accounted for some of the results, she said.