the range of plasma levels of a drug within which optimal therapeutic effects occur. Suboptimal effects may occur both below and above the therapeutic window. Evidence for true therapeutic windows was never well established; perhaps the best evidence existed for the tricyclic antidepressant nortriptyline. Few modern psychotropic drugs require therapeutic monitoring, although lithium is a notable exception; it has a very narrow therapeutic range below which it has no therapeutic effect and above which adverse effects and toxicity dominate.