In pharmacology, what does it mean for a drug to have a narrow therapeutic index?

• A. The effective and toxic doses are far apart
• B. The effective and toxic doses are very close together
• C. The drug is highly effective for many patients
• D. The drug can be prescribed safely in high doses

Answer: (B)

Having a narrow therapeutic index means that the effective dose of a drug and its toxic dose are very close together. This characteristic is critical in pharmacology because it indicates that there is a small margin of safety between doses that produce the desired therapeutic effect and doses that may lead to adverse effects or toxicity.

For drugs with a narrow therapeutic index, careful monitoring of drug levels and patient response is necessary to minimize the risk of toxicity. This ensures that patients receive an effective dose without crossing into the range that could cause harm. Examples of such drugs include warfarin and digoxin, where even small changes in dosage or drug levels can lead to significant clinical consequences.

In contrast, options suggesting that effective and toxic doses are far apart imply greater safety margins, and choices referring to general effectiveness or high dosing safety do not specifically address the significance of the therapeutic index.