What is the best treatment for the depression phase of bipolar disorder?
There is no single answer, except that in all cases a person with bipolar depression needs to be taking a mood stabilizer to prevent mania. Sometimes psychotherapy and gently waiting can then help a depressive episode to resolve without further medication.
If depression persists, the big decision then is which people with this condition can get better with only a mood stabilizer and which people also need to take a traditional antidepressant, always combined with a mood stabilizer. Mood stabilizers, in theory, work on both mania and depression. These include lithium, valproic acid, carbamazepine, and second-generation antipsychotics such as olanzapine, quetiapine, aripiprazole, risperidone, and others. In practice these all tend to be better at treating and preventing mania more than depression, though some individuals have a truly "bimodal" response with relief of both phases of illness.
Another mood stabilizer, lamotrigine, is effective for bipolar depression for some patients, but does not work as well to prevent manic episodes. Many experts recommend a combination of lamotrigine and lithium or one of the others as a reasonable first option. If this does not help depression, then combining one of the strong anti-manic drugs with a traditional antidepressant - an SSRI, bupropion, venlafaxine, etc. - is used.
However, while the mood stabilizer reduces the risk of being flipped up into mania it does not eliminate the risk. Therefore, after response to an antidepressant, many doctors prefer to reduce and slowly stop the antidepressant to see if the mood stabilizer alone works for the long haul. But the key is never to use JUST an antidepressant for bipolar depression. Always use a mood stabilizer in conjunction.