Bipolar II disorder (BP-II; pronounced "type two bipolar disorder") is a bipolar spectrum disorder (see also Bipolar disorder) characterized by at least one episode of hypomania and at least one episode of major depression. Diagnosis for bipolar II disorder requires that the individual must never have experienced a full manic episode (unless it was caused by an antidepressant medication; otherwise one manic episode meets the criteria for bipolar I disorder). Symptoms of mania and hypomania are similar, though mania is more severe and may precipitate psychosis. The hypomanic episodes associated with bipolar II disorder must last for at least four days. Commonly, depressive episodes are more frequent and more intense than hypomanic episodes. Additionally, when compared to bipolar I disorder, type II presents more frequent depressive episodes and shorter intervals of well-being. The course of bipolar II disorder is more chronic and consists of more frequent cycling than the course of bipolar I disorder. Finally, bipolar II is associated with a greater risk of suicidal thoughts and behaviors than bipolar I or unipolar depression. Although bipolar II is commonly perceived to be a milder form of Type I, this is not the case. Types I and II present equally severe burdens.
Bipolar II is difficult to diagnose. Patients usually seek help when they are in a depressed state. Because the symptoms of hypomania are often mistaken for high functioning behavior or simply attributed to personality, patients are typically not aware of their hypomanic symptoms. As a result, they are unable to provide their doctor with all the information needed for an accurate assessment; these individuals are often misdiagnosed with unipolar depression. Of all individuals initially diagnosed with major depressive disorder, between 40% and 50% will later be diagnosed with either BP-I or BP-II. Substance abuse disorders (which have high comorbidity with BP-II) and periods of mixed depression may also make it more difficult to accurately identify BP-II. Despite the difficulties, it is important that BP-II individuals are correctly assessed so that they can receive the proper treatment. Antidepressant use, in the absence of mood stabilizers, is correlated with worsening BP-II symptoms.