Penicillins: allergy screening, first-dose observation and finishing the course

One class, several groups

All beta-lactams bind and inactivate enzymes that bacteria need to build their cell walls. The penicillins include natural penicillins such as penicillin G and penicillin V, aminopenicillins such as amoxicillin and ampicillin, penicillinase-resistant drugs such as nafcillin and dicloxacillin, and piperacillin with tazobactam. Adding clavulanate, sulbactam or tazobactam protects the drug from some bacterial beta-lactamases.

Route details matter. Benzathine penicillin G is intended for deep intramuscular injection only, forming a depot that releases drug over days to weeks, so check the route carefully. Most penicillins are cleared in urine and need dose reduction in severe renal impairment, and very high doses can cause seizures, particularly when kidney function is poor. Food does not affect amoxicillin absorption, which helps with adherence at home.

Taking an allergy history that means something

Most penicillin adverse effects are hypersensitivity reactions. Immediate reactions include anaphylaxis, urticaria and angioedema, while delayed reactions include serum sickness, rashes and exfoliative dermatitis, which usually appears after seven to ten days. Ask which drug was taken, what happened, how quickly it started and what treatment was needed, then document the answer precisely.

Many reported reactions are not truly allergic, such as nausea or vague symptoms, and most people who report penicillin allergy do not react on re-exposure. The risk is still higher after a genuine previous reaction. Skin testing and desensitisation are specialist decisions. A rash during amoxicillin treatment for infectious mononucleosis is often nonallergic, but report it rather than reclassifying it yourself.

Cross-reactivity questions with cephalosporins

Cross-reactivity between penicillins and cephalosporins is less common than once believed, about 2 percent of penicillin-allergic patients in MSD Manual figures. Risk depends on the side chain of the specific cephalosporin, and cefazolin has very low cross-reactivity. Even so, patients who have had a significant reaction to one beta-lactam need caution with others.

For exam questions, the safe nursing action is to clarify before giving a cephalosporin to a patient with a history of penicillin anaphylaxis, reporting exactly what the reaction was. Do not automatically refuse every cephalosporin for a vague or mild history either. The prescriber and pharmacist weigh the reaction details against the alternatives.

First-dose observation and other adverse effects

Anaphylaxis can cause death within minutes, so observe closely during and after a first dose, especially a parenteral one, according to local policy. Signs include a swollen mouth, throat or tongue, difficulty breathing, severe dizziness and widespread itchy rash. If they appear, stop the infusion, stay with the patient, call for help and follow the emergency protocol.

Gastrointestinal upset is common, and severe or bloody diarrhoea can signal Clostridioides difficile infection. Less common effects include interstitial nephritis, haemolytic anaemia, leukopenia, which is seen most often with nafcillin, and thrombocytopenia. Intramuscular injections can be painful, and repeated use of the same intravenous site can cause thrombophlebitis. Probenecid raises penicillin blood levels.

Teaching and a worked example

Teach patients to take the medicine until the prescription is finished, even if they feel better, because stopping early or skipping doses can leave infection incompletely treated and encourage resistance. They should seek emergency care for swelling or breathing difficulty, report rashes and severe diarrhoea, and tell every clinician about any penicillin reaction.

Consider a hypothetical patient ten minutes into a first dose of intravenous piperacillin with tazobactam who reports throat tightness, has hives and a falling blood pressure. Options include giving an oral antihistamine and continuing, slowing the infusion, or stopping it and activating the emergency response. Stopping and escalating is correct, because slowing still delivers the allergen and an antihistamine alone does not treat airway or circulatory compromise.

Sources and further reading

MSD Manual Professional: Penicillins. Penicillin groups, benzathine for deep IM use only, renal dosing, hypersensitivity types, re-exposure risk, mononucleosis rash and other adverse effects.

MSD Manual Professional: Cephalosporins. Penicillin-cephalosporin cross-reactivity of about 2 percent, side-chain dependence and low cefazolin cross-reactivity.

NHS: Phenoxymethylpenicillin. Signs of anaphylaxis and angioedema, emergency action and serious diarrhoea.

MedlinePlus: Penicillin V potassium. Teaching to finish the prescription and the risk of incomplete treatment and resistance from stopping early.