There are classes of mentally ill who are in that condition through no fault of their own. There are some mentally ill people who have engaged in no illicit substance abuse. This subject may be broken down by the fact that illicit substance use among the mentally ill is sometimes described, even by legitimate researchers, as "self-medication". The weakness of this approach is that the practice is not medically supervised. (A bipolar/manic-depressive friend of mine who is currently in a locked facility, uses the strongest coffee, something that doesn't help him.) The unsupervised self-administration of psychotropic substances has the accompanying danger of over-use and addiction.
Everyone has "downs" and "blue days" regardless of organic mental illness. Illicit substance use can be defined, in opposition to the "self-medication" belief/excuse, as Self-Induced Mental Illness.
Another distinction worthy to be pursued, involves the fact that some mental illnesses are described as having a fully "organic" cause. This distinction seems to be centered treatment: Some conditions, such as certain obsessive-compulsive cases, can be cured by an antibiotic regimen. Some bipolar/manic-depressive cases can also be shown to have a clearly biologic component. This distinction therefore addresses treatability.
This type of thinking has the limitation, that other cases of mental illness aren't as easily shown NOT to have an organic component. It could lead to the idea, like the old attitude that some poor people are "unworthy", that some mentally ill people are worthy and some are not. (I once had a relative who plainly stated that "people are poor because they want to be that way". This person later became so poor that he was unable to afford to eat or take necessary medications. He later died, alone and without help.) Anyone is capable of undergoing disability. Does the experience of having to wear glasses or take medications help us to identify more closely with the disabled?
This leads into the subject of attitudes, especially prejudices and shame regarding psychological disability. There is a demonstrable tendency to regard mental illness as in some ways a "communicable disease" that should be quarantined, as if all mentally ill people present some kind of danger to the "non-mentally ill".
I once had a small dog which developed cancer. A larger dog would nip at the small, ill dog. The little dog dug a little escape area under the foundation of the house where it could escape the harassment of the larger dog. The explanation of the behavior seems to be that in dog pack-society, an ill dog must be fed and cared for; the "pack" has a tendency to chase away the ill, non-productive member.
This bears on the subject of utilitarianism, an actual philosophy of ruthless exclusion of dependent disabled people which fails to recognize their intrinsic worth as persons with immortal souls created in the Image and Likeness of God.