That seems a bit deceptive by the researchers. I don't think anyone is saying "short term unemployment" leads to opioids. I think most people assume long term unemployment or lack of long term opportunities lead to hopelessness and drug use.

Certainly there is a difference between I got laid off from Company A and it took be 3 months to land a job at Company B and the only major employer in my area shut down and I have no real prospects.

I may be mistaken, but isn't "lack of participation in the labor force" equivalent to long term unemployment rather than short term unemployment?

Isn't the question whether there is a causative relation between "long term unemployment" and "opioid use"? Short-term unemployment has no bearing on the issue.