Why does a drug with a narrow therapeutic index require careful dosing?

A drug with a narrow therapeutic index requires careful dosing because the margin of safety between the dose that produces the desired effect and the dose that causes dangerous side effects is very small.1

In clinical practice, this margin is often referred to as the therapeutic window—the concentration range above which a drug becomes toxic and below which it is ineffective. Because this window is relatively small for certain medications, even minor variations in how an individual metabolizes or excretes a drug can lead to toxicity or therapeutic failure.2

To ensure patient safety, clinicians must be precise when prescribing such drugs. For medications like phenytoin, digitalis, and lithium, it is common practice to monitor plasma levels to maintain the drug within the narrow range where it remains both safe and effective. This approach adheres to the fundamental principle of drug therapy: using the lowest effective dose to minimize the risk of adverse reactions.2