I generally trust people with physical disabilities to "self-medicate". If someone with a wheelchair says that something will help them live like a normal person, I'm inclined to think they're telling the truth.

But an awful lot of mental health issues are ones where the people tend to make terrible choices when it comes to dealing with their mental health (not every disease, but an awful lot these days). A lot of these disabilities are sustained and enabled by poor coping strategies (e.g. avoiding things they don't like, acting in destructive ways, expecting and getting special treatment, etc).

What about the cases where allowing people to use their mental health as an excuse is making their mental health worse?