21% [1] of homeless have a serious mental illness that predates their living situation, of which an even smaller chunk is schizophrenia. The major reasons for homelessness are predominantly financial (also [1]). We think that the sequence of events is mental illness/drugs → homelessness → poverty, but the true sequence is an averse financial event + lack of support → homelessness → mental illness (as a result of the homelessness) → drugs to cope.
We saw the most pronounced examples of this in Houston's homeless program, which has a 90% success rate by providing housing first [2]. For many, having a safe home is the only resource needed to get clean.
Homelessness is torture. Anyone in that situation would turn to drugs to cope. Demonizing the drugs is flying straight over the cause of the drugs themselves. If you want to fix homelessness, give homeless people homes.
[1]: https://www.samhsa.gov/blog/addressing-social-determinants-h...