Lithium is a mood stabilizer with a narrow therapeutic index used primarily for bipolar disorder. The therapeutic serum level is 0.6-1.2 mEq/L, and toxicity occurs when levels exceed 1.5 mEq/L. The difference between therapeutic and toxic doses is very small, making careful monitoring essential. Lithium is absorbed in the GI tract, is NOT protein-bound, and is eliminated entirely by the kidneys. It competes with sodium for reabsorption in the proximal renal tubule. Any condition reducing sodium or fluid (dehydration, vomiting, diarrhea, sweating, low-sodium diet, diuretics) causes the kidneys to reabsorb more sodium AND more lithium, raising levels to toxic ranges. Toxicity progresses through predictable stages. Mild (1.5-2.0): nausea, vomiting, diarrhea, fine-to-coarse tremor, drowsiness, weakness. Moderate (2.0-2.5): confusion, ataxia, slurred speech, blurred vision, muscle twitching, hyperreflexia. Severe (>2.5): seizures, coma, cardiac arrhythmias, cardiovascular collapse, death. The most common scenario is chronic toxicity: a stable patient develops a precipitating event (GI illness, new medication, worsening kidneys). Common drug interactions: NSAIDs (increase lithium 15-20%), ACE inhibitors, ARBs, thiazide diuretics -- all reduce lithium clearance.