side by side comparison

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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Comparison

Side-by-side2 compared
Dimension
Penicillins
Cephalosporins
Class & mechanism
  • Inhibit bacterial cell-wall synthesis
  • Inhibit bacterial cell-wall synthesis
Indications
  • Strep, syphilis
  • broad gram-positive cover
  • ★Surgical prophylaxis
  • broadens by generation
  • 3rd gen then gram-neg, meningitis/CNS
Route & forms
  • PO or IV (amoxicillin, pen G, pip-tazo)
  • PO or IV (cephalexin, cefazolin, ceftriaxone)
Key assessment
  • Assess penicillin/cephalosporin allergy first
  • Assess penicillin/cephalosporin allergy first
  • ★Penicillin allergy then cross-reactivity risk
Monitoring
  • Renal dosing
  • monitor renal function
  • Renal dosing
  • monitor renal function
Adverse effects
  • GI upset, diarrhea, C. diff risk
  • GI upset, diarrhea, C. diff risk
  • ★Disulfiram-like reaction w/ alcohol (cefazolin)
Anaphylaxis / safety
  • Anaphylaxis — report rash, throat swelling
  • Anaphylaxis — report rash, throat swelling
  • Ceftriaxone: avoid IV calcium in neonates
Contraindications & interactions
  • Prior severe beta-lactam allergy/anaphylaxis
  • Prior severe beta-lactam allergy/anaphylaxis
Patient teaching
  • Complete the full prescribed course
  • Complete the full prescribed course
  • Avoid alcohol during and 3 days after
Class & mechanism

Penicillins

  • Inhibit bacterial cell-wall synthesis

Cephalosporins

  • Inhibit bacterial cell-wall synthesis
Indications

Penicillins

  • Strep, syphilis
  • broad gram-positive cover

Cephalosporins

  • ★Surgical prophylaxis
  • broadens by generation
  • 3rd gen then gram-neg, meningitis/CNS
Route & forms

Penicillins

  • PO or IV (amoxicillin, pen G, pip-tazo)

Cephalosporins

  • PO or IV (cephalexin, cefazolin, ceftriaxone)
Key assessment

Penicillins

  • Assess penicillin/cephalosporin allergy first

Cephalosporins

  • Assess penicillin/cephalosporin allergy first
  • ★Penicillin allergy then cross-reactivity risk
Monitoring

Penicillins

  • Renal dosing
  • monitor renal function

Cephalosporins

  • Renal dosing
  • monitor renal function
Adverse effects

Penicillins

  • GI upset, diarrhea, C. diff risk

Cephalosporins

  • GI upset, diarrhea, C. diff risk
  • ★Disulfiram-like reaction w/ alcohol (cefazolin)
Anaphylaxis / safety

Penicillins

  • Anaphylaxis — report rash, throat swelling

Cephalosporins

  • Anaphylaxis — report rash, throat swelling
  • Ceftriaxone: avoid IV calcium in neonates
Contraindications & interactions

Penicillins

  • Prior severe beta-lactam allergy/anaphylaxis

Cephalosporins

  • Prior severe beta-lactam allergy/anaphylaxis
Patient teaching

Penicillins

  • Complete the full prescribed course

Cephalosporins

  • Complete the full prescribed course
  • Avoid alcohol during and 3 days after

★ marks the fact that sets a column apart.

Clinical Pearl

Anaphylaxis to penicillin = hold all cephalosporins. Rash only = cephalosporins usually safe — assess the reaction type.

Component Topics

CephalosporinsPenicillins — Use & Allergy

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