A client asks the nurse to explain the meaning of a narrow therapeutic index of a medication. Which information should the nurse use to answer the question?

The therapeutic index of a medication is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes toxicity. When a drug has a narrow therapeutic index, there is less of a margin of safety; the difference between a therapeutically beneficial dose and a harmful, potentially toxic dose, is small.

So, if a medication has a narrow therapeutic index, it means that the range between its helpful effect and harmful effect is quite small. This makes the safe and effective dosing of the drug more challenging, requiring careful monitoring to avoid drug toxicity.

For example, if a drug has a narrow therapeutic index, the patient might be at risk of toxicity if the dose is slightly increased, or might not receive enough therapeutic benefit if the dose is slightly decreased. Therefore, drugs with a narrow therapeutic index often require regular blood tests to ensure that the drug is maintained at a safe level in the body.