A 69-year-old man with bipolar disorder is on lithium carbonate and bendroflumethiazide. He presents with confusion and slurred speech; lithium toxicity is suspected. Which finding is most likely to be observed?

• A. Slurred speech
• B. Nausea
• C. Coarse tremor
• D. Drowsiness

Answer: (C)

Lithium toxicity presents with neurotoxic signs, and a key interaction here is that bendroflumethiazide raises lithium levels by promoting its reabsorption in the kidneys (often with volume depletion). When lithium is elevated, the most characteristic early finding is a coarse tremor, reflecting CNS irritation of motor pathways. Nausea, drowsiness, and even slurred speech can occur with toxicity, but the tremor is the typical, most sensitive clue in this scenario. The thiazide–lithium interaction compounds the risk, so the patient’s confusion and slurred speech fit toxicity, with the coarse tremor being the most likely observable sign.