Periods of high or irritable moods are called "manic" episodes. The person becomes very active, but in a scattered and unproductive way, sometimes with painful or embarrassing consequences. Examples are spending more money than is wise or getting involved in sexual adventures that are regretted later. A person in a manic state is full of energy or very irritable, may sleep far less than normal, and may dream up grand plans that could never be carried out. The person may develop thinking that is out of step with reality — psychotic symptoms — such as false beliefs (delusions) or false perceptions (hallucinations). During manic periods, a person may run into trouble with the law. If a person has milder symptoms of mania and does not have psychotic symptoms, it is called a "hypomanic" episode.
Bipolar disorder is now divided into two subtypes (bipolar I and bipolar II). Bipolar I disorder is the classic form where a person has had at least one manic episode. In bipolar II disorder, the person has never had a manic episode, but has had at least one hypomanic episode and at least one period of severe depression. Bipolar II disorder may be more common than bipolar I. A third disorder, closely related to bipolar disorder, is cyclothymia — people with this disorder fluctuate between hypomania and mild or moderate depression.
The vast majority of people who have manic episodes also experience periods of depression. If manic and depressive symptoms overlap for a period, it is called a "mixed" episode. In some people, moods alternate rapidly or it is difficult to tell which mood — depression or mania — is more prominent.
People who have one manic episode most likely will have others if they do not seek treatment. The illness tends to run in families. Unlike depression, in which women are more frequently diagnosed, bipolar disorder happens nearly equally in men and women. Bipolar I and II disorders occur in up to 4% of the population.
The most important risk of this illness is the risk of suicide. People who have bipolar disorder are also more likely to abuse alcohol or other substances.