CYP3A4 Inhibitors — FRCA Primary MCQ

Correct answer: E — Erythromycin

Erythromycin is the CYP3A4 inhibitor. By reducing CYP3A4-mediated metabolism, it can increase exposure to susceptible substrates, including anaesthetically relevant drugs such as midazolam and alfentanil. The magnitude of an interaction depends on the substrate and route of administration, so the unqualified term “potent” is less precise than identifying erythromycin as an inhibitor. Phenytoin, rifampicin, carbamazepine and St John's wort induce CYP3A4. They generally increase enzyme expression and therefore reduce the plasma concentration and duration of action of CYP3A4 substrates, although induction develops and resolves over time rather than occurring immediately.

Reference: Panpharma UK Ltd. Erythromycin 1 g Powder for Solution for Infusion, Summary of Product Characteristics, section 4.5: Interaction with other medicinal products and other forms of interaction. Revised 2025. https://www.medicines.org.uk/emc/product/7311/smpc