What does a narrow therapeutic index indicate about a drug?

A narrow therapeutic index indicates that there is a small margin between the minimum effective dose and the minimum toxic dose of a drug. This means that there is a fine line between achieving the desired therapeutic effect and causing adverse effects or toxicity. Because of this limited window, even slight variations in drug concentration can lead to serious safety concerns.

For drugs with a narrow therapeutic index, it is critical to closely monitor the patient's response and the drug levels in the bloodstream to ensure they remain within the therapeutic range. Therefore, these medications typically require more vigilant monitoring than those with a wider therapeutic index, where the margin for safety is broader.

This characteristic emphasizes the importance of understanding pharmacokinetics and pharmacodynamics for drugs with a narrow therapeutic index, as the potential for higher risks of toxicity is significant.