I have the book "Bipolar II" by Dr. Ronald Fieve. He was one of the first doctors to test and prescribe lithium in the USA in the 70's. He states that lithium is the gold standard for BP, just like Rose said. He also states that if you suffer from psychosis and lithium does nothing for you, you're probably schizophrenic. (pg. 187). He states than in this way, lithium can be a diagnostic tool. If it helps you, you have BP. If it doesn't help you, you have something else. Lithium is specific to BP.
The question therefore is: does lithium by itself help you?