If people haven't harmed anybody, then they themselves shouldn't be harmed, even if professionals believe it is help. Removing that barrier opens the door to arbitrary treatment and involuntary commitment, the likes of which not seen since the 19th century.
Mental drugs cause a lot of effects, including causing other problems. They usually cause you to feel like you're in a cloud of anti-emotion. It really sucks when you want to feel stuff. Worse yet, are other side effects like tardive dyskinesia and tremors and things. And those, once you get them, don't go away.
What I gather, the only really humane way to handle mental health problems relating to homelessness is to medically arrest them, have a panel of doctors whom medicate according to the mental disease, wait until the drugs take effect (upwards to 6 weeks), and then explain what's going on and how they wish to continue. If the patient wants to stay on drugs, awesome: they should be provided free of charge. Along with that, governmental services they may have turned down prior can be reactivated and put into motion to assist them.
Or... if they don't want help/treatment, they should sign a paper stating that they were medicated, informed, and that they wish to not be on drugs and live on the street. At this time, they are released to live as they choose. And this "medical arrest" to inform them of their rights would never happen again. That's a one-time affair.
This "Arrest" would happen once, and would be done to provide informed consent. We already acknowledge that things like contracts and the like can't be made while on drugs, alcohol, or under duress. There's no reason why mental defects that cause effects like drugs can't also be grouped in with this.
This sounds to be the most fair..
Sounds like most drivers I know...