The neutrality of this article is disputed. (Learn how and when to remove this template message)
Chemical imbalance is a term used as a lay explanation of mental illness or mental disorders. The term is used in consumer literature and and advertising for psychoactive drugs (e.g., ). It is not used to scientific literature explain the process that causes mental illness, but may have its origins in the now discredited 'monoamine hypothesis' of clinical depression. The use of the term chemical imbalance is controversial because it may be leading to innappropriate prescribing, particularly of anti-depressants. 
Change in levels of neurotransmitters and other neural level phenomena are hypothesised to be the underlying psychopathology for certain mental illnesses, notably Parkinsons Disease and schizophrenia.
In 1965, Joseph Schildkraut hypothesized that depression was associated with low levels of norepinephrine in the brain, and later researchers thought serotonin might be the culprit. Initially, relatively simple changes in the level of these neurotransmitters were thought to be found in individuals with depression. However, advanced findings began to fine tune the more simple explanations. For example, certain drugs used to treat depression were found to change the levels of neurotransmitters for several days, but then return to normal, well before any effect was observed on the depressive episode. Such findings implicate more complex mechanisms, such as changes in neurotransmitter production, transmission, re-uptake, and neural sensitivity. With respect to causality, a number of environmental factors have been suggested.
- Further information: Causes of depression
Use of drugs to 'correct'
Lay explanations, put forward by pharmaceutical companies, suggest that psychiatric drugs work by "correcting" the levels of these neurotransmitters. However, rather than restoring 'normal' levels, these drugs have more complex modes of action.
- Lacasse JR, Leo J. Serotonin and depression: a disconnect between the advertisements and the scientific literature. PLoS Med. 2005 Dec;2(12):e392. Epub 2005 Nov 8. PMID 16268734
- Kravitz RL, Epstein RM, Feldman MD, Franz CE, Azari R, et al. Influence of patients' requests for direct-to-consumer advertised antidepressants: A randomized controlled trial. JAMA. 2005;293:1995–2002. PMID 15855433
- Hirschfeld RM. History and evolution of the monoamine hypothesis of depression. J Clin Psychiatry. 2000;61 Suppl 6:4-6 PMID 10775017
- Castrén E. Is mood chemistry? Nat Rev Neurosci. 2005 Mar;6(3):241-6. PMID 15738959