Pharmacy Technician Certification Board Practice Exam

Which of the following medications has a narrow therapeutic index?

1. Xarelto

2. Phenytoin

3. Fluoxetine

4. Lisinopril

The correct answer is: Phenytoin

Phenytoin is considered to have a narrow therapeutic index because the range between its effective dose and toxic dose is quite small. For medications with a narrow therapeutic index, monitoring of drug levels is critical to ensure that the dosage stays within the therapeutic range. This close margin means that small changes in the dose can lead to efficacy or toxicity, making it vital for healthcare providers to regularly monitor the patient’s drug levels, as well as their clinical response and side effects. In contrast, the other medications listed have a wider therapeutic index, meaning that there is a larger range of dosages that can be safely taken without leading to toxicity or ineffective treatment. Therefore, while all medications should be managed carefully, the specific characteristics of phenytoin necessitate more stringent monitoring and dosage adjustments to avoid adverse effects and ensure therapeutic effectiveness.