The LA Times interviewed the author of "Dreamland," the 2015 book about the rise of OxyContin addiction (he has a new book):
I believe that’s most definitely what’s happened. In 2008, the Mexican government decided to make ephedrine [a stimulant long used in making meth] effectively illegal. And the trafficking world had to switch to another form [known as the P2P method]. By 2012, 2013, tests [on seized meth] from the DEA [Drug Enforcement Administration] showed that it was all being made with P2P.
What only now is becoming clear is that P2P meth carries with it profound symptoms of schizophrenia, primarily paranoia and hallucinations. The ephedrine meth was a party drug, big in the gay community. Kind of a euphoric thing. But now this meth puts people into their own brains. There’s not a lot of that wanting to be with other folks.
So when you’re driving through L.A. and see people who are naked, people crawling on the ground, what you see is P2P meth?
Yes. Also when I see big stacks of bicycle frames, shopping carts. This stuff seems to breed hoarding in a spectacular way. It seems to turn people mad and incapable of living with other people, very quickly.
The related article is this one in The Atlantic:
[She] has been a physician for homeless people in Los Angeles since 2003. She noticed increasing mental illness—schizophrenia, bipolar disorder—at her clinics around the city starting in about 2012. She was soon astonished by “how many severely mentally ill people were out there,” [she] told me. “Now almost everyone we see when we do homeless outreach on the streets is on meth. Meth may now be causing long-term psychosis, similar to schizophrenia, that lasts even after they’re not using anymore.”