What does a narrow therapeutic index of a medication indicate?

A narrow therapeutic index indicates that there is a small margin between the effective dose of a medication and the dose at which it becomes toxic. This means that a slight increase in dosage or changes in plasma levels can lead to adverse effects or toxicity, making it critical to monitor patients closely when they are on such medications. Medications with a narrow therapeutic index require careful dosing and frequent monitoring of plasma levels to ensure safety and efficacy.

For instance, medications like warfarin or lithium are known for having a narrow therapeutic index, and this necessitates regular blood tests and adjustments based on individual response and changes in metabolism. This concept is crucial in pharmacology as it underscores the importance of patient-specific factors in medication management.