What does a "narrow therapeutic index" indicate about a drug?

A narrow therapeutic index indicates that there is little difference between the therapeutic dose and the toxic dose of a drug. This means that the range of doses at which the drug is effective (therapeutic) is very close to the range at which it could cause harmful effects (toxic). As a result, drugs with a narrow therapeutic index require careful monitoring to ensure that patients receive the correct dosage that is effective but not harmful.

Drugs in this category often necessitate frequent blood tests to monitor plasma levels, as small changes in dosage can lead to significant variations in therapeutic and toxic effects. Understanding this concept is crucial for managing medications effectively, especially in situations where precise dosing is essential to avoid adverse effects or therapeutic failures.