Penicillin allergy

Penicillins are among the most useful and frequently prescribed antibiotics, however as with all medicines they can cause adverse reactions. These include allergic reactions ranging from mild rash to life threatening anaphylaxis.

Allergy is one of the most common and important adverse effects of penicillin and related drugs such as amoxicillin (including co-amoxiclav), flucloxacillin and piperacillin and can occur in 1-10% of exposed individuals.

Anaphylaxis is rare, with an estimated frequency of 1-5 per 10,000 courses administered, but can be fatal. Furthermore the chemical structure of cephalosporins (cefalexin, cefuroxime etc) is similar to that of penicillins and cross-sensitivity can occur in up to 10% of patients.

Penicillins are often the cornerstone of treatment for serious infections and sepsis in the hospital setting. If a patient has a documented penicillin allergy, alternative antibiotics will need to be prescribed. This could require use of quinolones or cephalosporins, with higher risk of C. difficile infection, or gentamicin or vancomycin, which are nephrotoxic and ototoxic. Optimal management of the patient may be compromised if a patient has been wrongly attributed with a penicillin allergy.

All available drug sensitivity issues should be recorded. It is important to clarify and record the nature of the reaction. Check with the patient and the medical notes prior to all prescribing. Please do not label a patient as being allergic to an antibiotic on the basis of side effects of a drug (e.g. nausea, diarrhoea etc.)

Type 1 reaction – Immediate anaphylaxis (IgE mediated)

Patients with a history of immediate hypersensitivity following administration of penicillin, recognisable by features of urticaria, laryngeal oedema, bronchospasm, hypotension or local swelling within 72 hours of administration, should not receive a penicillin.

Patients who are truely allergic to one penicillin will be allergic to all because the hypersensitivity is related to the basic penicillin structure.

Patients with a history of immediate hypersensitivity to penicillin may also react to the cephalosporins and other beta-lactam antibiotics. They should not receive these antibiotics.

Type 2 reactions – Delayed reaction (non-IgE mediated)

More commonly penicillin hypersensitivity manifests as a rash, the typical presentation being a maculopapular, erythematous rash symmetrically disposed over the legs, buttocks and trunk.

Patients with a definite history of non-urticarial rash allergy to penicillin should not receive a penicillin but the likelihood of serious cross-sensitivity with cephalosporins or carbapenems is very low so other non-penicillin beta lactam antibiotics can be used in these patients.

Very rarely penicillins can cause pemphigus vulgaris or pemphigoid-like reactions. Penicillins and cephalosporins should not be prescribed to these patients.

Patients often describe side effects such as diarrhoea or nausea as ‘allergies’, so careful history taking is extremely important to distinguish between true allergy and manageable side effects. Similarly patients reporting minor rashes restricted to small areas of the body, or who develop rashes more than 72 hours after exposure, probably do not have genuine hypersensitivity. For serious infections for which penicillins are the preferred treatment, vague histories of such reactions do not contra-indicate penicillin use. Discuss with microbiology if necessary.

It is also worth noting that maculo-papular rashes can also occur in patients treated with either ampicillin or amoxicillin who have concomitant viral infections such as glandular fever. Such reactions are not allergic phenomena and do not contra-indicate future use of these or related drugs.

Penicillin Allergy Risk Colour Coding

For patients with a type 1 hypersensitivity reaction to penicillin,

In life threatening infections such as bacterial meningitis, consider using 3rd generation cephalosporins even in patients with a history of penicillin allergy.