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Bipolar disorder is divided into four categories:

• Bipolar I disorder. The most common type of bipolar disorder. This diagnosis requires at least one manic or mixed episode (mania accompanied by some depressive features). Episodes of major depression usually (but not always) occur. Men and women are equally affected and episodes may be particularly severe or psychotic (with delusions or hallucinations) and may require hospitalization. This category is divided into six separate disorders, depending on whether the patient has experienced only one (single episode) or multiple episodes and, in the case of multiple episodes, the nature of the last episode (hypomanic, manic, mixed, depressed or unspecified).
  
  
• Bipolar II disorder. At least one episode of hypomania and one episode of major depression must have occurred for this diagnosis. Further, the patient must have never had a manic or mixed episode. Hypomanic episodes do not lead to psychosis or require hospitalization, though depressive episodes may. Bipolar II disorder is more common in women than in men and may be difficult to diagnose because many patients do not view hypomanic episodes as a cause for concern. They often do not remember them during depressive episodes without reminders from others. This disorder may develop into bipolar I disorder if a manic episode occurs.
  
  
• Cyclothymia. Cycling between hypomania and lows in mood that do not meet the criteria for major depressive episodes. These milder episodes may be less severe and/or shorter in duration. They are, however, chronic and last for at least two years with no lapse in symptoms for more than two months at a time. This disorder may develop into bipolar I disorder if a manic episode occurs or into bipolar II disorder if a major depressive episode occurs.
  
  
• Nonspecified bipolar disorder. Bipolar disorder with features that do not match those noted above. This may be more common in early onset bipolar disorder.