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Erythromycin Phenytoin Interaction — ORE Part 1 MCQ

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HardPharmacology & TherapeuticsErythromycin Phenytoin InteractionORE Part 1

A patient taking long-term phenytoin requires an antibiotic for an acute dental infection. Erythromycin is being considered because of immediate penicillin hypersensitivity. Which statement is most appropriate?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: D — Avoid erythromycin because phenytoin may reduce erythromycin exposure and erythromycin may increase phenytoin concentrations.

Explanation lettering: C = shown as A · E = shown as C · A = shown as D · D = shown as E

**A is correct.** The UK erythromycin SmPC states that phenytoin may induce erythromycin metabolism, causing sub-therapeutic erythromycin concentrations and decreased effect; it also lists phenytoin among medicines whose serum concentrations may increase with erythromycin. This is clinically important because phenytoin has saturable metabolism, so inhibition can produce disproportionate rises in concentration and toxicity risk. The phenytoin SmPC confirms that it is mainly metabolised by CYP2C9/CYP2C19 and is a potent enzyme inducer. B is incomplete because it ignores the potential increase in phenytoin concentration. C reverses the expected phenytoin effect. D denies a documented interaction. E wrongly attributes enzyme induction to erythromycin; phenytoin is the inducer.

Reference: Electronic Medicines Compendium. Erythromycin Tablets BP 250 mg, Summary of Product Characteristics, section 4.5 Interactions, accessed July 2026. https://www.medicines.org.uk/emc/product/502/smpc