The marijuana industry has relied heavily on a 2014 study that showed a decrease in opioid-related deaths correlating with medical marijuana legalization. A 2019 study that allowed for more time to pass from the passage of legalization to the implementation of legalization found that the opposite was true, with a 23% increase in opioid deaths. 

More importantly, the study also found that because medical marijuana users represented such a small portion of the population, there was no way that they could exert such an impact on the opioid epidemic. Subsequent studies have confirmed the finding.

Read the study here: https://www.pnas.org/content/116/26/12624

A 4-year prospective study in the highly respected journal, The Lancet Public Health, followed patients with chronic non-cancer pain and found no evidence marijuana use mitigated pain severity or interference or that marijuana affected rates of opioid prescribing or opioid discontinuation (Campbell, et al., 2018)

Read the study here: https://www.ncbi.nlm.nih.gov/pubmed/29976328