Erythromycin QT Prolongation — MFDS Part 1 MCQ

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HardPharmacologyErythromycin QT ProlongationMFDS Part 1

During medicines reconciliation before erythromycin is prescribed, which concurrent medicine makes co-administration contraindicated specifically because pharmacokinetic and pharmacodynamic interactions increase the risk of QTc prolongation and torsades de pointes?

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Correct answer: E — Domperidone

Domperidone is correct. Both medicines can prolong QTc, while erythromycin also inhibits CYP3A4-mediated metabolism of domperidone. The domperidone SmPC reports an approximately threefold rise in exposure and a mean QTc increase of 9.9 ms during co-administration; the combination is contraindicated because of ventricular arrhythmia and torsades de pointes risk. Hydroxychloroquine also raises cardiovascular concern, but the erythromycin SmPC specifies careful benefit–risk assessment rather than the same categorical interaction. Simvastatin is contraindicated for a different reason—increased statin exposure and rhabdomyolysis. Warfarin may have an enhanced anticoagulant effect, and theophylline concentrations may rise, requiring monitoring or dosage adjustment rather than avoidance specifically for QT prolongation.

Reference: Electronic Medicines Compendium, Domperidone 1 mg/ml Oral Suspension SmPC, sections 4.3–4.5, revised 4 November 2025. https://www.medicines.org.uk/emc/product/3188/smpc