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Flucloxacillin vs Amoxicillin — SCE Infectious Diseases MCQ

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EasyAntimicrobial StewardshipFlucloxacillin vs AmoxicillinSCE Infectious Diseases

A 65-year-old man develops an infected diabetic foot ulcer with surrounding cellulitis. The GP prescribes Flucloxacillin 500 mg QDS. The patient asks why Flucloxacillin rather than Amoxicillin. What is the key pharmacological difference?

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Correct answer: A — Flucloxacillin is resistant to staphylococcal penicillinase (beta-lactamase) — it covers S. aureus, which is the most common cause of skin/soft tissue infection; Amoxicillin is inactivated by staphylococcal beta-lactamase

Flucloxacillin is a narrow-spectrum isoxazolyl penicillin designed to resist staphylococcal penicillinase (beta-lactamase). It is the treatment of choice for MSSA skin/soft tissue infections. Amoxicillin (an aminopenicillin) has broader Gram-negative coverage but is inactivated by staphylococcal beta-lactamase — making it unreliable for S. aureus infections. Understanding the beta-lactam subclasses (aminopenicillins, isoxazolyl penicillins, carboxypenicillins, ureidopenicillins) and their spectra is fundamental to rational antibiotic prescribing.

Reference: NICE NG141 2019 – SSTI; BNF 2024 – Penicillin pharmacology