chemical imbalance

The sense "mental illness" is due to the fact that some mental illnesses are caused by chemical imbalances in the brain.

chemical imbalance (plural chemical imbalances)

1. Used other than figuratively or idiomatically: see chemical,‎ imbalance.
2. (euphemistic) Mental illness.
   • Warden, Rob and Drizin, Steven (2009) True Stories of False Confessions, →ISBN, 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.”

• chemically imbalanced