A patient taking which drug should be instructed to maintain a constant and adequate salt intake?

• A. Methotrexate
• B. Lithium
• C. MMR
• D. Sodium Valproate

Answer: (B)

Lithium levels are tightly linked to sodium balance in the kidneys. Sodium and lithium compete for reabsorption in the renal tubules, so when sodium intake is reduced or a patient becomes dehydrated, more sodium is reabsorbed to maintain volume and, incidentally, more lithium is reabsorbed too. That raises lithium blood levels and increases the risk of toxicity. Keeping a constant and adequate salt intake helps stabilize how the kidneys handle lithium, reducing fluctuations in its level. It’s especially important to avoid sudden changes in salt intake or dehydration, which can precipitate toxicity. Other drugs listed don’t rely on this sodium‑balance mechanism in the same way.