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Pharmacology · single question

Which antibiotic is CONTRAINDICATED in a patient taking a calcium channel blocker?

  1. Cephalexin
  2. Amoxicillin
  3. Clarithromycin
  4. Azithromycin
Show answer and explanation

Correct answer: C. Clarithromycin

  • A. Cephalexin is renally cleared and does not interact with calcium channel blockers.
  • B. Amoxicillin is not metabolized by CYP3A4 and has no meaningful interaction with calcium channel blockers. It is a safe choice here.
  • C. CORRECT. CLARITHROMYCIN is a potent inhibitor of CYP3A4, the enzyme that metabolizes dihydropyridine calcium channel blockers (amlodipine, nifedipine, felodipine). Blocking that pathway causes CCB levels to climb, producing profound hypotension, shock and acute kidney injury. Population studies have documented hospitalization and excess mortality from precisely this pairing in older adults.
  • D. Azithromycin is the SAFE macrolide in this setting — it is a negligible CYP3A4 inhibitor, which is exactly why it is the recommended alternative when a macrolide is needed in a patient on a CCB.

Note: Memorize the split within the macrolides: ERYTHROMYCIN and CLARITHROMYCIN are strong CYP3A4 inhibitors; AZITHROMYCIN is not. The same inhibition raises levels of statins, warfarin, benzodiazepines and cyclosporine.

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