Like bizi said it is a mood stabilizer and is used to stabilize both mania and depressive moods. You may require additional medication in conjunction with the lithium or some other medication all together. Bipolar depression is a bit different than just depression in how medication works on it. A lot of people with bipolar find that antidepressants don't work at all, work only mildly, or send them into an episode. I take both a med that is used as a mood stabilizer plus an antidepressant.