When I seem bitter, insulted and angered by the suggestion of CFS/ME being a mental illness, it is not because I don't believe that mental illness is a "real" illness.
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My sentiments exactly. Several people close to me have depression and other mental illnesses. Anti-depressants work fine for them. They just make me sicker because I don't have depression.
For example, cortisol levels are off in one direction for depression and the opposite direction for CFS. Treating my cortisol level with something intended for depression will simply send it further out of whack.
Exercise will make those with depression feel better; it'll make a person with CFS feel worse. When I took my doctor's advice to take a walk every day, every day I collapsed into bed longer after that walk, until one day I couldn't stand up at all -- I'd run myself into energy bankruptcy, and crumpled to the floor when I tried to get out of bed. You'll never find a depressed person crawling to the bathroom because they can't stand; most CFS patients have that experience at least once.
Continuing to class CFS as a "mental illness" allows them to avoid doing the research into physical causes that would find the proper and appropriate treatments that would get us back to work. Because we can't work while extremely sick, that provides further support for the misguided argument that we are "lazy". I know lazy people -- they won't get off the couch. They definitely won't get up and do things that push them to the point of collapse. I used to clean till I collapsed, till I read a few things about CFS that scared me -- long-term physical consequences of overdoing. Now I keep my enthusiasm under control; I do what I can, but I don't push it. As a result, my health has been steadily improving instead of going further downhill.