“Studies Show”: Marijuana Legalization and Opioid Deaths

In his NY Times column today, Ross Douthat argues that legalizing marijuana is a big mistake. Douthat makes a number of arguments, but let me focus on one point he makes in the column: that recent research suggests legalizing marijuana increases opioid deaths. This point is made in just one sentence of the essay, so let me quote it in full:

There was hope, and some early evidence, that legal pot might substitute for opioid use, but some of the more recent data cuts the other way: A new paper published in the Journal of Health Economics finds that “legal medical marijuana, particularly when available through retail dispensaries, is associated with higher opioid mortality.”

Kudos to Mr. Douthat for actually linking to the paper. That’s what the internet is for! Yet so many writers in traditional news sources fail to do this.

Now, on to the paper itself. There is nothing untrue in what Douthat writes. First, there was plenty of “early evidence” that legalizing marijuana reduced opioid deaths. More on this in a moment. And the study he cites by Mathur and Ruhm is particularly well done. It is published in the top health economics journal. But the main point of the paper is to say “we think the rest of the literature is wrong, and we’re going to try really hard to convince you that we are right.”

What does the rest of this literature say? Here’s a brief tour (all of these are cited in Mathur and Ruhm). The variable in question is opioid deaths.

Bachhuber et al. (2014): 25% reduction in deaths after medical marijuana legalization

Shover et al. (2019): 21% reduction in deaths up to 2010, but this reverse to a 23% increase from 2010-2017

Powell, Pacula, and Jacobson (2018): 20% reduction in deaths from medical marijuana legalization, though the effect becomes statisically insignificant when adding 2010-13 data, which the authors say is because “states have become more stringent in their regulation of dispensaries” — also a 28% reduction in deaths for states with retail dispensaries

Chan, Burkhardt and Flyer (2020): 20-35% reduction in deaths from recreational marijuana legalization

Smith (2020): 11% reduction in county-level deaths following the opening of a dispensary

Hsu and Kovács (2021): 17% reduction in county-level deaths following the opening of a dispensary

Just reading this set of papers, it would seem like there is a lot of evidence that legalizing marijuana reduces opioid deaths.

But Mathur and Ruhm (the paper cited in the NYT column) have a new approach and newer data. They pick up on some things suggested in the papers by Shover and Powell, particularly the reversal of the relationship between legalization and opioid deaths, as well as some problems they saw in the other model specifications.

So who is right? Or perhaps more crucially, why did the relationship seem to flip after about 2010? Mathur and Ruhm, of course, think they are right. And Douthat seems to think Mathur and Ruhm are correct. No offense to Douthat — it’s not his job — but I doubt he can explain the modeling differences between the papers. He seems to have just picked the paper that fits his views (or more charitably, he just picked the paper with the most recent publication date).

It’s encouraging to see the latest scientific research cited in the country’s most influential newspapers. But we should be careful about changing course on public policy based on a single paper, however well done. The research on this will continue, and in fact is continuing.

Here’s another very recent paper by Alvaro et al., which slices the data a bit more finely. They find a few things (using data up through 2020): yes, medical marijuana laws in general are associated with more opioid deaths, but medical legalization with dispensaries was associated with fewer deaths, and furthermore recreational legalizations were also associated with fewer deaths. This paper is also not the last word! But it can help guide states to provide them better information on exactly how legalization should happen, if a state choses to legalize.

As a final word, a topic like this is one that economists love to study. The variation in policy over time and across states allows for lots of data, as well as lots of choices about modeling. This will continue to be debated in the literature, but public policy will move forward to some extent independent of what the best, most recent research says. That’s just how it goes.

We also shouldn’t always be so narrowly focused on one outcome, though it is the nature of good research that this will often be the case. The case for legalizing or not legalizing marijuana was never solely about whether it reduced opioid deaths (indeed, opioid deaths were barely on the radar during the early debates on legalization). It’s important to consider the totality of effects of prohibition versus legalization (including different varieties of legalization).

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