Lithium carbonate provides a source of lithium ions, which compete with sodium ions at various sites in the body.

Lithium is used to even out the highs (mania) and lows (depression) in mood associated with of bipolar disorder. In some cases, Lithium is used to treat people with depression even though they have never experienced a manic episode. Many of these patients have not responded adequately to antidepressants alone, so the addition of Lithium to an antidepressant is tried for some people.

Lithium has also been used to treat people with schizophrenia in cases where changes in thinking happen at the same time as a mood change that looks like either mania or depression.