They should read the latest (Nov. 2017) from the nih =>
Recreational Cannabis Legalization and Opioid-Related Deaths in Colorado, 2000-2015.
CONCLUSIONS: Legalization of cannabis in Colorado was associated with short-term reductions in opioid-related deaths. As additional data become available, research should replicate these analyses in other states with legal recreational cannabis.
https://www.ncbi.nlm.nih.gov/pubmed/29019782