“If you believed the results of the first study, it’s hard to argue that you don’t believe the results of the second one, since the methods are the same,” says Chelsea Shover, a postdoctoral researcher at Stanford University and an author of the new PNAS study.
Why did such similar methods produce such a starkly different result? For one thing, at the time of the first study, only 13 states allowed medical cannabis, and they were mostly in the American West, where the opioid epidemic was slow to take hold, Shover says. But by 2017, 47 states had a medical-cannabis law, and the opioid epidemic had touched pretty much every corner of the country. The nature of the epidemic also changed, as deaths from heroin and fentanyl overtook deaths from prescription opioids. The second study therefore included