I'm not sure where you get your information, but mental illnesses certainly can be identified by physical (neurochemical) differences from a statistical baseline. Probably one of the easier ones to identify is dopamine deficiency and dysthymia.

Drugs can certainly mess up someone's life (see my earlier post). But untreated mental illness can literally end your life (either directly, via suicide, or indirectly, via medical complications from body dysmorphic conditions)