What does it mean for a drug to have a narrow therapeutic index?

A drug with a narrow therapeutic index means that the difference between the minimum effective concentration and the minimum toxic concentration is very small. This implies that there is a fine margin for error in dosing; small increases in the dose can lead to toxic effects, while small decreases can result in ineffective therapy.

Drugs with a narrow therapeutic index require careful monitoring and precise dosing to avoid adverse effects and ensure efficacy. They often necessitate regular blood level checks and adjustments to the dosage based on individual patient factors, highlighting the importance of medical supervision while using these medications.

The other options do not accurately reflect the implications of a narrow therapeutic index. For example, a drug with a wide range of safe dosages would be considered to have a wide therapeutic index, not a narrow one. Describing a drug as fast-acting and requiring frequent dosing does not pertain specifically to its therapeutic index, nor does stating it is safe for use without medical supervision, as narrow therapeutic index drugs often require more stringent oversight to prevent toxicity.