Lithium

Lithium is the oldest mood stabilizer in modern use, and seventy years after its introduction it remains the gold standard for bipolar I disorder. There is no drug in psychiatry with a stranger profile: a simple ion, no well-characterized single receptor target, a narrow therapeutic window, substantial monitoring burden, and a property that no other psychotropic shares. Lithium reduces suicide. Not just suicide attempts — actual suicide deaths. The effect is real and replicated. No other mood stabilizer has shown this, and no other psychotropic has. That alone is reason to know lithium well.

The mechanism is genuinely incompletely understood. Lithium inhibits glycogen synthase kinase-3 (GSK-3β), modulates inositol monophosphatase, affects second messenger systems, has neurotrophic effects including BDNF and adult neurogenesis, and probably does several other things we don't yet have clean stories for. The clinical effects — antimanic, antidepressant in bipolar depression, prophylactic against future episodes, and anti-suicidal — appear to be downstream of multiple intracellular changes accumulating over weeks.

The therapeutic window is narrow. Therapeutic serum levels are 0.6 to 1.2 mEq/L. Acute mania often requires the higher end; maintenance often does well at 0.6 to 0.8. Above 1.5, mild toxicity begins — tremor worsens, GI symptoms, ataxia. Above 2.0, moderate toxicity — confusion, myoclonus, hyperreflexia. Above 2.5, severe toxicity — seizures, coma, death. The cliff is real, and the cliff can be reached unexpectedly when something else changes the drug's clearance.

Drug interactions are critical and they all share a pattern: anything that reduces renal lithium clearance raises the level. NSAIDs do this. ACE inhibitors and ARBs do this. Thiazide diuretics do this. Dehydration does this. The patient on stable lithium 1200 milligrams who develops back pain and starts ibuprofen can be in lithium toxicity within a week. The patient who develops a viral illness with vomiting and reduced fluid intake can be there in days. Counsel every lithium patient: NSAIDs are not safe; use acetaminophen. Alert every prescriber to the lithium. Maintain hydration. Watch for tremor, GI symptoms, confusion.

Monitoring is the infrastructure that makes long-term safe use possible. Baseline labs: renal function (BUN, creatinine, eGFR), thyroid function (TSH), electrolytes, pregnancy test in women of childbearing potential, ECG in patients over 50 or with cardiac risk factors. Annual repeat of all of these. Lithium level five to seven days after each dose change, then every three to six months at steady state. Trough levels — drawn approximately twelve hours after the last dose. Long-term lithium can cause chronic kidney disease and hypothyroidism in 15-30% of patients over time; monitoring catches these early.

For the patient with bipolar I disorder, lithium is the medication that has earned the longest evidence base. The anti-suicidal effect makes it particularly compelling in patients with prior suicide attempts. The cost is the monitoring burden and the patient education required. The benefit, for the right patient, is decades of stability that other agents cannot reliably match.

Lithium asks more of the clinician and the patient than any other psychotropic. For the right patient, what it gives back is unique.