in layman's terms...

bipolar disorder, characterized by fluctuating periods of intensely positive and negative affect (or mood), is a biological problem caused by a variety of neurochemical imbalances.

Depressive states are treated with antidepressants in the same way that unipolar, or major, depression is...

...but manic states, characterized by impuslivity, rapid speech and an overabundance of energy, can only be treated with lithium.