Beta-Lactam Antibiotics: Penicillins vs Cephalosporins — Cross-Allergy, Generation Differences
A patient says "I'm allergic to penicillin" and needs antibiotics. Holding all cephalosporins is overcautious and may delay treatment — but giving one after true anaphylaxis could be fatal. The NCLEX expects you to know which reaction type changes the decision.
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- Inhibit bacterial cell-wall synthesis
- Inhibit bacterial cell-wall synthesis
- Syphilis (pen G drug of choice)
- Strep throat, otitis media (amox)
- ★Surgical prophylaxis (cefazolin)
- Gram-neg cover ↑ with each generation
- Meningitis (ceftriaxone, 3rd gen)
- PO/IV: amoxicillin, pen G, pip-tazo
- PO/IV: cephalexin, cefazolin, ceftriaxone
- Ask about beta-lactam allergy first
- Ask about beta-lactam allergy first
- ★PCN allergy: ~1–2% cross-reactivity
- Renal dosing
- Monitor renal function
- Renal dosing
- Monitor renal function
- GI upset, diarrhea, C. diff risk
- GI upset, diarrhea, C. diff risk
- ★Alcohol → disulfiram-like rxn (cefotetan)
- Anaphylaxis — report rash, throat swelling
- Pen G benzathine: deep IM only, never IV
- Anaphylaxis — report rash, throat swelling
- Ceftriaxone: avoid IV calcium in neonates
- Prior beta-lactam anaphylaxis
- Prior beta-lactam anaphylaxis
- Complete the full prescribed course
- Complete the full prescribed course
- No alcohol during + 3 days after (cefotetan)
Penicillins
- Inhibit bacterial cell-wall synthesis
Cephalosporins
- Inhibit bacterial cell-wall synthesis
Penicillins
- Syphilis (pen G drug of choice)
- Strep throat, otitis media (amox)
Cephalosporins
- ★Surgical prophylaxis (cefazolin)
- Gram-neg cover ↑ with each generation
- Meningitis (ceftriaxone, 3rd gen)
Penicillins
- PO/IV: amoxicillin, pen G, pip-tazo
Cephalosporins
- PO/IV: cephalexin, cefazolin, ceftriaxone
Penicillins
- Ask about beta-lactam allergy first
Cephalosporins
- Ask about beta-lactam allergy first
- ★PCN allergy: ~1–2% cross-reactivity
Penicillins
- Renal dosing
- Monitor renal function
Cephalosporins
- Renal dosing
- Monitor renal function
Penicillins
- GI upset, diarrhea, C. diff risk
Cephalosporins
- GI upset, diarrhea, C. diff risk
- ★Alcohol → disulfiram-like rxn (cefotetan)
Penicillins
- Anaphylaxis — report rash, throat swelling
- Pen G benzathine: deep IM only, never IV
Cephalosporins
- Anaphylaxis — report rash, throat swelling
- Ceftriaxone: avoid IV calcium in neonates
Penicillins
- Prior beta-lactam anaphylaxis
Cephalosporins
- Prior beta-lactam anaphylaxis
Penicillins
- Complete the full prescribed course
Cephalosporins
- Complete the full prescribed course
- No alcohol during + 3 days after (cefotetan)
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Anaphylaxis to penicillin = hold all cephalosporins. Rash only = cephalosporins usually safe — assess the reaction type.
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