What is lithium ?

Lithium affects the flow of sodium through nerve and muscle cells in the body. Sodium affects excitation or mania.

Lithium is used to treat the manic episodes of manic depression. Manic symptoms include hyperactivity, rushed speech, poor judgment, reduced need for sleep, aggression, and anger. It also helps to prevent or lessen the intensity of manic episodes.
Lithium may also be used for other purposes not listed.

Im sure nirvana song its inspired by this drug, or maybe Kurt Cobain use this drug.