Educational use only. Drug cards are AI-assisted study material for NCLEX preparation.
Penicillins (penicillin G/V, amoxicillin, ampicillin, piperacillin)
Mechanism of action
Bind penicillin-binding proteins (PBPs) on bacterial cell walls and inhibit transpeptidation, blocking peptidoglycan cross-linking. Result: cell-wall lysis and bactericidal killing of susceptible organisms. Coverage varies by sub-class — narrow-spectrum (penicillin G/V) for streptococci and syphilis; aminopenicillins (amoxicillin, ampicillin) for some gram-negatives; antipseudomonal (piperacillin) for broader coverage including Pseudomonas.
Adverse effects
Life-threatening / NCLEX-tested
- Anaphylaxis — life-threatening; 5–10% of US patients report a penicillin allergy but most are not true IgE-mediated; clarify history carefully
- Stevens-Johnson syndrome / toxic epidermal necrolysis (rare)
- Severe diarrhea / Clostridioides difficile colitis
- Acute interstitial nephritis (more with methicillin/nafcillin)
- Hemolytic anemia with high-dose IV penicillin G
- Seizures with very high doses, especially in renal impairment
- Severe hyperkalemia with potassium-penicillin (large IV doses)
Side effects
Common — what to teach
- Diarrhea (especially with amoxicillin)
- Nausea
- Maculopapular rash (often non-allergic, especially with amox + EBV/mononucleosis)
- Yeast vaginitis or oral thrush
- Mild abdominal cramps
- Black hairy tongue (rare, harmless)
Food & drug interactions
Probenecid raises penicillin levels by blocking renal tubular secretion. Oral contraceptives may have reduced efficacy with antibiotics in general — counsel backup contraception during course. Methotrexate levels rise with penicillins — toxicity risk. Aminoglycosides synergize for serious gram-positive infections (give in separate lines — direct mixing inactivates aminoglycosides).
Nursing implications
Assessment, monitoring, patient teaching
- ALLERGY ASSESSMENT: clarify the reaction (rash vs anaphylaxis), timing, and severity; "PCN allergy" without anaphylaxis often means cephalosporins are safe — discuss with the team
- Always have epinephrine, diphenhydramine, and IV access available for first IV doses; observe for at least 30 minutes after first dose for anaphylaxis
- Take oral penicillin V on an empty stomach for best absorption; amoxicillin can be taken with or without food
- Complete the FULL prescribed course even if symptoms resolve — incomplete courses drive resistance
- Counsel on diarrhea — bloody, severe, or persistent diarrhea = call (rule out C. diff)
- For depot injections (benzathine penicillin G for syphilis): IM only, NEVER IV (cardiac arrest if given IV); use a large muscle, observe for 30 minutes
- Reconcile renal function — many penicillins need renal dose adjustment
When to hold / contraindications
- Documented IgE-mediated penicillin allergy (anaphylaxis, urticaria, angioedema)
- Severe rash, fever, mucosal involvement (suspect SJS/TEN/DRESS)
- Severe acute kidney injury (some require dose adjustment, others contraindicated)
- Active C. difficile infection until provider review
- Benzathine PCN G via IV route — never give IV
Memory anchor
Penicillins end in "-cillin." "30-minute observation, true allergy = STOP, depot = IM only." Amox + mono virus = nuisance rash, not anaphylaxis.
Practice Penicillins questions
Test your recall on real NCLEX-style pharmacology questions — with full rationales and an AI Coach for the parts you miss.