Pharmacology · single question
A patient presents with an acute abscess and is allergic to penicillin. Which antibiotic should you NOT prescribe?
Show answer and explanation
Correct answer: A. Cefalexin (Cephalexin)
A) Cefalexin — CORRECT (do NOT give this).
Cephalosporins (like cefalexin) share a beta-lactam ring structure with penicillins. There is approximately 1–10% cross-reactivity. In a patient with a known penicillin allergy, cephalosporins are relatively contraindicated, especially if the allergy is severe (anaphylaxis).
B) Clindamycin: First-line alternative for dental infections in penicillin-allergic patients. Safe to use.
C) Erythromycin: A macrolide — no cross-reactivity with penicillin. Alternative option.
D) Azithromycin: Also a macrolide — safe in penicillin-allergic patients.
Cephalosporins (like cefalexin) share a beta-lactam ring structure with penicillins. There is approximately 1–10% cross-reactivity. In a patient with a known penicillin allergy, cephalosporins are relatively contraindicated, especially if the allergy is severe (anaphylaxis).
B) Clindamycin: First-line alternative for dental infections in penicillin-allergic patients. Safe to use.
C) Erythromycin: A macrolide — no cross-reactivity with penicillin. Alternative option.
D) Azithromycin: Also a macrolide — safe in penicillin-allergic patients.