Formerly known as manic depression, this mental illness is believed to be caused by a combination of genetic factors and neurological functioning, according to the National Institute of Mental Health. It is characterized by unusually intense shifts in emotion, energy, behavior and activity levels in what are called “mood episodes.” Such episodes are usually classified as manic, hypomanic, depressive or mixed episodes. Bipolar disorder often develops during late adolescence or early adulthood.
Refer to someone as having bipolar disorder only if the information is central to the story and a licensed medical professional has formally diagnosed the person. Do not use bipolar as an adjective for something that rapidly or drastically changes.