If a patient taking lithium decreases their sodium intake to 2g/day, what is the likely effect on the drug's activity?

The correct answer emphasizes that when sodium intake is decreased, the activity of lithium may be enhanced. This effect occurs because lithium and sodium share similar pathways in the body, particularly in renal (kidney) function. Lithium is a monovalent cation, just like sodium. When sodium intake is reduced, the kidneys may retain more lithium to maintain electrolyte balance, leading to higher lithium levels in the bloodstream. This can enhance the drug's activity, potentially increasing its effectiveness in treating conditions like bipolar disorder.

This situation also highlights the importance of a balanced dietary sodium intake for patients on lithium therapy, as fluctuations can significantly affect drug levels and clinical response. Monitoring sodium intake is crucial to avoid complications and maintain therapeutic efficacy.