A 32-year-old woman on a therapeutic dose of lithium (serum level 0.9 mEq/L) for bipolar disorder develops coarse tremor, slurred speech, and confusion after starting a new drug. Which drug most likely precipitated lithium toxicity?
- A Metformin
- B Ibuprofen ✓
- C Atenolol
- D Fluconazole
Correct answer: B. Ibuprofen
Explanation
NSAIDs like ibuprofen inhibit renal prostaglandin synthesis, which reduces renal blood flow and GFR. Since lithium is entirely renally excreted (like sodium) by proximal tubular reabsorption, any reduction in GFR or sodium depletion triggers compensatory proximal tubular reabsorption of both sodium and lithium, raising plasma lithium levels into the toxic range. Thiazide diuretics cause a similar effect. Atenolol and metformin do not significantly affect lithium clearance.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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