Are medical cannabis laws linked to less opioid overdose mortality? Results from a new study

In 2014, a study by Bachhuber et al. showed that, in the USA, state-level medical cannabis laws were associated with lower-than-expected opioid overdose mortality rates from 1999 to 2010. A new study by Shover et al. using the same methodology, found that in the longer period the findings reversed. In fact, in both cases, the authors underline how this kind of study should not be used to make causal claims.