What does it mean when a drug has a narrow therapeutic index?

A drug with a narrow therapeutic index indicates that the range between its effective dose and its toxic dose is very small. This means that there is little margin for error in administering the medication; even slight variations in dosage can lead to an ineffective treatment or potentially toxic effects. For medications with a narrow therapeutic index, careful monitoring and precise dosing are crucial to ensure safety and effectiveness. Examples of such drugs include warfarin and lithium, where maintaining the appropriate concentration in the bloodstream is essential to avoid adverse effects. Therefore, understanding the implications of a narrow therapeutic index is vital for safe pharmacological practice.