Azithromycin Pharmacokinetics — MFDS Part 1 MCQ
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Correct answer: A — Greater tissue distribution and a much longer terminal half-life
Azithromycin undergoes extensive tissue and intracellular distribution and has a terminal half-life of approximately 68 hours, compared with about 2 hours for erythromycin. This prolonged persistence permits once-daily administration and, for some authorised indications, a 3-day regimen of 500 mg once daily. Erythromycin generally requires divided dosing because of its shorter half-life. Food does not consistently produce a clinically advantageous increase in azithromycin bioavailability, and its effect depends on formulation. Azithromycin also has clinically relevant interactions, including effects involving P-glycoprotein substrates, so these are not absent. High peak serum concentrations are not its principal pharmacokinetic advantage; the key feature is sustained tissue exposure. The alternative azithromycin regimen is 500 mg on day 1 followed by 250 mg daily on days 2–5, not only through day 3.
Reference: Electronic Medicines Compendium. Azithromycin 500 mg film-coated tablets, SmPC sections 4.2 and 5.2, revised 13 May 2026, https://www.medicines.org.uk/emc/product/2276/smpc; Erythromycin Tablets BP 250 mg, SmPC sections 4.2 and 5.2, revised 6 December 2025, https://www.medicines.org.uk/emc/product/502/smpc