A mental illness is caused by damage on certain parts of brain, resulting in disturbed functioning of the brain cells, such as abnormal transmission of the chemical among brain cells. Such disturbance inside the brain manifests as mental symptoms involving cognitive function, content of thought, perception, emotion, behaviour and biological function (e.g. sleep and appetite) and impaired overall functioning of the person.
Generally, psychiatric medications work by restoring balance of chemicals in brain. For example, an antipsychotic normalizes the transmission of a chemical known as dopamine, and therefore, makes delusion subside. Some medication might even promote the healing of brain. For example, an antidepressant works by restoring the normal transmission of a chemical called serotonin among the brain cells, as well as promoting the healing of damaged part of brain through a growth factor called brain-derived neurotrophic factor (BNDF).
Common types of psychiatric medications include antidepressants, antipsychotics, mood stabilizers, tranquilizers and hypnotics, and anti-dementia agents etc. However, this traditional way of classifying psychiatric medications is actually imprecise. For example, the medications used to treat depression are also useful in treating anxiety disorders. Mood stabilizers, including lithium and anticonvulsants, are the main-stream medications for bipolar affective disorders; however, the new generation antipsychotics are found to have therapeutic effect on bipolar affective disorder as well.