One of the most difficult things about psychiatry, as opposed to surgery, or immunology or, in fact, most medical disciplines, at least to me, is the fact that it's extremely unclear whether someone is actually sick or not, and there's no real way to tell. Drugs can be effective on people who are not necessarily mentally 'ill'; and drugs can not be effective on people who are mentally 'ill'.
Stoudemire's Clinical Psychiatry defines a mental disorder by saying "It is, in fact, impairment of function that distinguishes 'eccentric behavior' from psychiatric illness."
Perhaps this seems self-evident; well and so. The difficulty is that establishing when someone is impaired is also a questionable task. One of the best examples of this is Attention Deficit Hyperactivity Disorder, which may strike as many as 7% of children. However, all sufferers are struck with different severities of symptoms, from the crippling to the almost unnoticeable; and it tends to be more a matter of art than of science in determining how to deal with the issue.
The Diagnostic and Statistical Manual of Mental Disorders (DSM) attempts to clarify the issue, but in my opinion, rarely does. Rather than setting out specific itemizations, the details presented are merely rough guidelines.
Rather, you are left with a wide variety of people, especially Americans, in my experience, who believe they have mental disorders- without necessarily a severe impairment of function, for which they receive pharmaceutical support. Because these pharmaceuticals can alter the biology of 'healthy' people just as well as they can the 'ill', in fact, these people do undergo changes. Of course, it is then up for debate as to whether they are 'healthy' now as opposed to before.
Arguably, the only definition that matters is whether they are more functional after whatever treatment than before. In this respect, it may be that psychiatric treatment is nothing more than a lifestyle choice- albeit an expensive one with possibly drastic side effects.