Well, I've been saying since at least 2014 that surely the opioid (and related heroin) problem in the US is an indication that the libertarian idea of drug legalisation is fanciful and misguided, at best. (I could use stronger words, but I'm feeling generous today.)
I see that one German Lopez at Vox has explained at length why he would now agree with me, in his article entitled:
I used to support legalizing all drugs. Then the opioid epidemic happened.
Maybe in 10 years time he'll change his mind about marijuana legalisation, too.