Have you ever considered that it may be the other way around? That the horrors of living on the street (and "horrors" is an appropriate term here, you are fighting for survival every day; it is beyond the realm of comprehension of the housed) might be causing the mental illness and drug use, rather than the other way around?
If I want to get a homeless person off of drugs, it sure as crisps is not going to happen until they have a roof over their head. The core issue is the lack of affordable housing. That should be priority number 1.
If I want to get a homeless person off of drugs, it sure as crisps is not going to happen until they have a roof over their head. The core issue is the lack of affordable housing. That should be priority number 1.