Penicillins — Use & Allergy

A client says they're "allergic to penicillin" — but 90% of them actually aren't. Knowing who truly is allergic changes the entire antibiotic plan and could prevent anaphylaxis.

Penicillins (amoxicillin, ampicillin, nafcillin, piperacillin) kill bacteria by binding penicillin-binding proteins and disrupting cell wall synthesis, making them bactericidal. They remain first-line for streptococcal pharyngitis, otitis media, syphilis, and many community-acquired infections. The critical nursing concern is allergy. True penicillin allergy involves an IgE-mediated Type I hypersensitivity reaction: urticaria, angioedema, bronchospasm, or anaphylaxis occurring within minutes to an hour of exposure. A prior anaphylactic reaction is an absolute contraindication — no penicillin, and extreme caution with cephalosporins due to a 1–2% cross-reactivity risk (highest with first-generation agents). However, most self-reported penicillin allergies are not true IgE-mediated reactions; GI upset or a childhood rash often gets mislabeled as allergy. Before administering any penicillin, you verify the specific reaction history: what happened, how soon, and how severe. Epinephrine must be immediately available whenever penicillin is given to any client with a questionable allergy history. Monitor the client for at least 30 minutes after the first dose.

Key Distinctions

Don't confuse a true IgE-mediated allergy (hives, throat swelling, anaphylaxis within an hour) with a drug intolerance (GI upset, non-urticarial rash days later) — only the first contraindicates re-exposure. Cross-reactivity with cephalosporins is 1–2%, not the outdated 10% figure still seen in older references. Students often think all beta-lactams are absolutely contraindicated with penicillin allergy, but carbapenems carry a similar or slightly lower cross-reactivity risk, and aztreonam (a monobactam) has negligible cross-reactivity and is generally considered safe in penicillin-allergic patients.

Clinical Pearl

Always ask WHAT happened, not just IF they're allergic. "My stomach hurt" is not anaphylaxis — that distinction changes the entire treatment plan.