Which of the following would be a reason for caution when prescribing Lithium?

Caution when prescribing Lithium is particularly warranted in the presence of signs of dehydration. Lithium has a narrow therapeutic window, meaning the difference between an effective dose and a toxic dose is small. Dehydration can lead to an increase in lithium levels because the body may not effectively eliminate the drug when there is less fluid available. This can result in toxicity and serious side effects.

In patients showing signs of dehydration, it is crucial to monitor hydration status closely and ensure that they are adequately hydrated before initiating or continuing lithium therapy. Dehydration can also affect renal function, further complicating lithium dosing and increasing the risk of adverse events.

The other considerations, like recent weight gain, a high sodium diet, or a low exercise level, do have implications for overall health and may influence treatment planning, but they do not pose an immediate risk in the same way that dehydration does regarding lithium administration.