The sense "mental illness" is due to the fact that some mental illnesses are caused by chemical imbalances in the brain.
- Used other than as an idiom: see chemical, imbalance.
- (euphemistic) Mental illness.
- Warden, Rob and Drizin, Steven (2009) True Stories of False Confessions, ISBN 0810126036, page 28: “Anyone who knew him well would suspect he had a chemical imbalance. He had been unhappy recently. And there was a history of suicide in the family; his mother had tried to kill herself.”