There is no single cause. The disorder tends to run in families which suggests that there is a genetic link. In people predisposed to the disorder, the onset can be triggered by stressful life events and the use of drugs of abuse. In rare cases individuals who for reasons such as a family history may be vulnerable to develop bipolar disorder, episodes may be triggered by the use of some older antidepressants and other medications including steroids.

An imbalance in various neurotransmitters (chemicals by which the brain cells communicate) may also be involved. There may also be disturbances in the production or release of certain hormones within the brain that contribute to causing bipolar disorder.