"Catherine was his sister, and on lithium"

Lithium carbonate (often known just as lithium) and other lithium salts are used as psychiatric medication. They are mainly used to treat the psychiatric condition called bipolar disorder (previously known as manic depression) but may also be used for depressive episodes in isolation.

Lithium is used to treat both the manic and depressive phases of bipolar disorder. It has been used as a mood stabiliser for about fifty years, although its mechanism is not fully understood. The lithium dosage needs to be very carefully monitored as it can be toxic if too much is taken.