Furosemide × Lithium

Furosemide (loop diuretic) lowers circulating volume and, via subclinical volume depletion, increases lithium reabsorption in the proximal tubules. Serum lithium rises by 25–40%.

Tremor, muscle twitching, nausea, diarrhoea, confusion, ataxia. Severe cases include seizures, coma, and nephrogenic diabetes insipidus with dehydration. Symptoms emerge when serum lithium exceeds 1.2 mmol/L.

Reduce lithium dose by 25–30% when starting or escalating furosemide; check serum lithium at 1 week, then monthly. Maintain normal sodium and fluid balance: deficits worsen lithium toxicity.