A condition (also called manic depression) characterized by extreme mood swings that alternate between depression and mania (a state of exaggerated elation and euphoria).

According to the National Institutes of Mental Health (NIMH), bipolar disorder affects an estimated 2 million Americans (roughly 1% of the population). The disease usually begins in the teens or early 20s and affects males and females equally. Children with attention deficit/hyperactivity disorder (ADHD) are at above-average risk for bipolar disorder: about one in four has or will develop it. Within a 10-year period, a person whose bipolar disorder is not treated with medication usually goes through an average of four depression/mania episodes. However, the pattern varies widely: mood swings can occur as frequently as four or more times a month, or as seldom as once every five years.

Bipolar disorder is the depressive illness considered most likely to have a biological...