Supply and demand for fentanyl

The FT has an article with the following headline and subhead:

Fentanyl deaths are falling. What’s behind the decline?

The evidence points to changes in the drug supply

It’s an excellent article, but there’s really no evidence to support the subhead (which is usually written by the editor, not the journalist.)

The article does provide evidence that there has been a recent decline in the quantity of drugs seized, but that could reflect either lower supply or lower demand.  To avoid “reasoning from a quantity change”, you need to look at contemporaneous changes in both price and quantity.  Is a rising price of fentanyl deterring users, or is less fentanyl demand resulting in lower prices?

Another article suggests that at least in Minnesota it might be the latter:

In recent months, the price of buying fentanyl on the street in the Twin Cities metro fell to $1-$2 a pill, a longtime narcotics investigator for the Hennepin County Sheriff’s Office confirmed.

That’s down from a roughly $20 price point a couple of years ago, Hennepin County Sheriff’s Major Rick Palaia said, adding that’s still about the cost far outside the metro. . . .

“Whether supply is going down or not, at least the deaths are going down,” Palaia concluded. “And we’ll continue to educate, and we’ll continue to try and take as much fentanyl off the street as we can.”

The FT article suggests what seems to me to be the most plausible explanation for the recent decline in fentanyl deaths, which began on the East Coast:

A third possibility is the idea put forward by Nabarun Dasgupta and colleagues at the US academic collective Opioid Data Lab, that much like an infectious disease epidemic, fentanyl has now worked its way through its susceptible population. Some died, others figured out ways to use without overdosing, so the remaining fentanyl-naive drug-using population has shrunk. The wave-style dynamics at work here mean it would also theoretically fit the east-west pattern.