Mental illnesses may be caused by a bad mixture of
chemicals (
neurotransmitters, etc.) in the
brain.
Drugs like
Prozac are supposed to restore your
out-of-whack brain stew to something more amenable to your
mental health and welfare.
Other drugs, like
cocaine,
crystal meth, etc. temporarily bind to
receptors in your brain, producing their respective
euphoric effects. Those drugs, however, are generally not designed with long-term safe use in mind, and being a long-time user may leave you permanently
whacked-out. Even
marijuana, when ingested in
mass quantities over a long period of time, will alter your brain's
chemistry (arguably not for the better...).