Which of the following is a key characteristic of lithium use?

Lithium use is associated with the necessity of monitoring electrolyte levels, particularly sodium. This is crucial because lithium can affect kidney function and the body's fluid balance, making individuals more susceptible to fluctuations in electrolyte levels. Adequate sodium levels are critical for preventing lithium toxicity, as lithium can become more concentrated in the body if sodium levels drop. Regular monitoring helps ensure that the patient remains within the therapeutic range and avoids potential adverse effects.

Other aspects of lithium usage, such as its potential for weight gain, safety across all populations, and classification as an over-the-counter medication, do not apply. Weight gain can occur but is not a defining characteristic of its usage. Additionally, lithium is a prescription medication that is not considered safe for all populations, particularly those with certain medical conditions or those who are pregnant. Thus, the need for electrolyte level monitoring is a critical aspect of effective and safe lithium therapy.