I’ve talked before here about the issues with confusing correlation and causation, and more recently I’ve also talked about the steps needed to establish a causal link between two things.

Thus I was interested to see this article in the Washington Post recently about the attempts to establish a causal link between access to medical marijuana and a decrease in painkiller related deaths. There had been studies suggesting that access to medical marijuana was associated with lower rates of overdose related deaths since this JAMA paper was published in 2014, and those findings were repeated and broadened in 2015 with this paper. Both papers found increased access to medical marijuana reduced painkiller related deaths by up to 25% over states with no such access. This showed at least some promise of moving towards a causal link, as it established a reproducible consistent association.

This was not without it’s critics. When the Washington Post covered the story about the 2015 paper, they interviewed a skeptical researcher who pointed out that painkiller users are at higher risk for overdose when they use medical marijuana as well. Proponents of medical marijuana pointed out that this only studied those who were prescribed painkillers. If it could be established that access to medical marijuana reduced the number of painkiller prescriptions being written, then you could actually start to establish a plausible and coherent theory….2 more links on the chain of causality.

Long story short, that’s what this new paper did. They took a look at how many prescriptions your average physician wrote in states with legal medical marijuana vs those without, and found this:

As a balance, they also looked at other drugs that had nothing to do with medical marijuana (like antibiotics or blood thinners) and discovered there was no difference in those prescription rates.

While the numbers for anxiety and depression medication are interesting, they may only translate in to a handful of patients per year. That pain medication number on the other hand is pretty damn impressive. 1,826 doses of painkillers could actually translate in to at least half a dozen patients per physician (if you’re assuming daily use for a year) or more if you’re assuming less frequent use. This gives some pretty hefty proof that medical marijuana could be lowering overdose rates by lowering the number of patients getting a different painkiller prescription to begin with.

I’d be interested to see if there’s a dose response relationship here….within the states that have legal medical marijuana, do states with looser laws/more access see even lower death rates? And do those states with the lower overdose death rates see an increase in any other death rates, like motor vehicle accidents?

Interesting data to ponder, especially since full legalization is on my state ballot this November. Regardless of the politics however, it’s a great example of how to slowly but surely make a case for causality.