“On label” means that a medication has received FDA approval for the prescribed usage. In order to receive an on-label indication a medication must be shown to be better than a placebo (sugar pill). FDA approval in no way implies that the medication is better, or worse, than other medications used for the same condition(s).
Some examples: Lithium and lamotrigine have FDA approval for the treatment of the mania and depression associated with bipolar disorder, as well as for maintenance (ongoing) treatment for mood stabilization. Other medications, which some studies have found to be effective in the treatment of mood swings, such as oxcarbazepine (Trileptal), and carbamazepine (Tegretol) are frequently used in an off label manner in treating bipolar disorder. And, to further confuse things, some medications have specific FDA-approved indications (uses) in bipolar disorder, such as for the treatment of acute mania, but do not have the indication for maintenance therapy; valproic acid (Depakote) is one example.