Why is Lithium considered a drug with narrow therapeutic index?

Lithium is considered a drug with a narrow therapeutic index because the difference between its effective dose and toxic dose is small. This means that only a slight increase in dosage can lead to toxicity, which can result in serious side effects, including renal impairment and neurological issues. Consequently, careful monitoring of blood levels is essential to ensure that patients remain within the therapeutic range. The variability in individual responses to lithium further complicates its use, necessitating personalized treatment plans.