Increasing the amount
of serotonin in depressed patients does not always improve
their mood. Some of them will also need more norepinephrine.
Still other patients may require a different kind of medication
(for example, a mood stabilizer such as lithium).
And even within a family of similar antidepressant medications,
some patients will do better with one than with others.
Every case of depression thus seems to represent a unique
kind of disturbance in the functioning of the brain,
so that several different medications often have to be
tried before one is found to be effective.