A mood disorder is a mental illness characterized primarily by mood swings or an abnormally high or low mood. Most mood disorders are defined and diagnosed by the occurrence of one or more mood episodes, or periods of abnormal happiness or sadness. Such episodes are not mood disorders in themselves.

Mood disorders include major depression, dysthymia and bipolar disorder, among others. Major depression and dysthymia involve periods of sadness and lack of pleasure or interest in normal activities as well as fatigue. Bipolar disorder includes episodes of both depression and mania, which is an elevated mood of (euphoria), combined with increased self-esteem, racing thoughts and reckless behavior or impulsive activity. If severe, episodes of high and low mood can involve psychotic symptoms, such as hallucinations and delusions.

The causes of mood disorders are not completely understood. However, a chemical imbalance in the brain seems to play a major role in their development. Brain chemicals called neurotransmitters convey messages between the nerves. Abnormalities in the regulation of these neurotransmitters are believed to cause alterations in mood. Mood disorders also appear to be linked to genetics. Individuals who have relatives with a history of a mood disorder have a greater risk of developing a mood disorder.

The diagnosis of a mood disorder involves a physical examination by a physician and a mental health evaluation by a physician or a mental health professional. Mood episodes or mood disorders can be triggered or imitated by other illnesses, substance abuse or side effects of medications, so a physician must rule out these potential causes first. The mental health evaluation includes a complete history of symptoms, including when they began, how long they have lasted and how severe they are. It is also noted whether the patient has experienced these symptoms before and, if so, whether and how they were treated.

Depressive mood disorders are typically treated with antidepressants. Mood stabilizing drugs, particularly lithium, are the primary medications for bipolar disorder. Patients with mood disorders may also benefit from different types of therapy, including psychotherapy and cognitive behavior therapy (CBT), in addition to medications. Electroconvulsive therapy (ECT) may be used in severe cases or in cases where medications are not effective or are not safe to use (e.g., pregnancy).