**Core Concept:** Monitoring serum lithium levels in patients on lithium therapy is essential to ensure therapeutic effect and prevent toxicity. Lithium is used in the treatment of bipolar disorder and other psychiatric conditions.

**Why the Correct Answer is Right:** Lithium levels should be checked at least twice during the initial phase of therapy, when the dose is being titrated, and then periodically (usually every 1-2 weeks) as the dose is stabilized. The rationale behind this is to ensure that the lithium concentration is within the therapeutic range (0.6-1.2 mEq/L) to achieve the desired therapeutic effects, such as stabilizing mood, preventing mood episodes, and improving psychotic symptoms.

**Why Each Wrong Option is Incorrect:**

A. Lithium level monitoring at admission only is incorrect because the initial dose titration phase requires frequent monitoring to adjust the dose effectively.

B. Lithium level monitoring every 6 months is not sufficient as lithium levels need to be checked more frequently during the initial phase and when dose adjustments are being made.

C. Lithium level monitoring when the patient experiences side effects is too late, as it might lead to toxicity and complications. Monitoring should be done proactively, especially during dose titration and when the dose is stable.

D. Lithium level monitoring only when the patient becomes euthymic (in a stable mood state) is not recommended. Lithium therapy should be monitored regardless of the patient's mood state, as lithium levels need to be checked during initiation and titration of the therapy, as well as when therapeutic adjustments are made.

**Clinical Pearl:** Regular monitoring of serum lithium levels ensures patient safety and effective treatment. Establish a routine of checking lithium levels at least twice during the initial phase (when titrating the dose) and then periodically (every 1-2 weeks) once the dose is stabilized. This practice helps prevent lithium toxicity and ensures optimal therapeutic benefits for the patient.