The neurological effect of the lithium ion Li+ makes some lithium salts useful as a class of mood stabilizing drugs, mostly for bipolar disorder. Therapeutically useful amounts of lithium (about 0.6 to 1.2 mmol/l) are only slightly lower than toxic amounts (>1.5 mmol/l), so the blood levels of lithium must be carefully monitored during treatment to avoid toxicity.
An important point that is often ignored is that at much lower doses, lithium is actually an important and natural nutrient. It helps protect brain function against injury from neurotoxins, improves mood, decreases aggressiveness and even has anti-viral properties. It is usually left out of multi-vitamins because of confusion between its use as a nutrient at very low dose (5-10 mg a day) and its use as a medication with significant side effects at 30-90 mg (300-900 mg of lithium carbonate) daily.