Item #3 in a Cato Institute video mentions evidence that overdose rates are declining in states (USA) where marijuana is legal, with little other interpretation evident than this: availability of marijuana reduces the number of people who are introduced to opioids for pain relief for a legitimate medical issue and who later become addicted to opioids, and thus fewer people later turn to illicit opioid markets where they are more likely to obtain laced products with higher potential to kill them.
Save lives. Improve health. Legalize weed. Or, at a minimum in jurisdictions where this is politically impossible at present, facilitate or even promote prescription access for pain relief purposes.