Ask the experts: are you truly allergic to penicillin?

Penicillin is one of the most important antibiotics we can prescribe for chest, throat and skin infections. The advantages of penicillin are that it is safe to use during pregnancy and breastfeeding, and is well tolerated in children. It is found in benzylpenicillin V, which is given as an injection. It is loosely related to amoxicillin, an oral antibiotic. A familiar antibiotic may be co-amoxiclav/augmentin, which contains amoxicillin and clavulanic acid. But some people are allergic to the drug. Symptoms may vary from a mild skin rash to a severe chain reaction within the body called anaphylaxis. Anaphylaxis cases caused by true penicillin allergy occurred at a rate of one to five per 10,000 cases of penicillin treatment, according to a review paper by Sanjib Bhattacharya of the Bengal School of Technology which was published in the Journal of Advanced Pharmaceutical Technology and Research in 2010. There is no published data on rates of penicillin allergy in Hong Kong. Two of Hong Kong's leading allergy specialists, Dr Lee Tak-hong, head of the Allergy Centre at the Hong Kong Sanatorium and Hospital, and Dr Adrian Wu Young-yuen, head of the Centre for Allergy and Asthma Care, have not found an increased prevalence of penicillin allergy in any specific ethnic population. Both Lee and Wu agree there is no evidence to confirm hereditary factors play a major role - if one of your parents is known to have a penicillin allergy, this does not necessarily increase your chances of developing it. If you are allergic to penicillin, doctors are obliged to err on the side of caution and note it on your medical record. This is because even when the initial allergic response is relatively minor, subsequent exposure to the antibiotic can trigger a severe anaphylactic reaction which can be fatal. If the doctor is unable to prescribe penicillin for you, this can restrict your choices, and less effective antibiotics may need to be used to deal with common infections. In cases such as recurrent tonsillitis, I rotate between different antibiotics to reduce the chances of drug resistance, where the antibiotic simply loses effectiveness. If the patient has a smaller number of non-penicillin-based antibiotics to select from, resistance may arise more easily. A 2001 study in JAMA, led by Dr Alan Salkind of the University of Missouri-Kansas City School of Medicine, said only 10 to 20 per cent of patients who self-reported penicillin allergy were found to be truly allergic. Patients I have seen over the years have reported an allergy to penicillin in a number of ways: • An event reported by a parent because they were too young to remember it. • The antibiotic causing a relatively mild reaction such as vomiting and diarrhoea. • An anaphylactic reaction causing a severe skin rash and serious symptoms within an hour of taking the antibiotic. There are two main types of allergic drug reactions, depending on the onset of presentation. With the immediate type, the reaction usually occurs within an hour of exposure and comprises an itchy skin rash called hives, swelling of the lips and tongue, difficulty breathing, dizziness and chest palpitations, abdominal pain and diarrhoea. These reactions on first exposure to penicillin are not life-threatening, but can be on subsequent exposure, as the immune response can unleash a more severe attack. The delayed type of reaction can occur on day seven of treatment, and even a few days after completing the course of antibiotic. The rash that appears can manifest in different forms. Therefore, it is not easy for doctors, let alone the patient, to tell whether they have a genuine penicillin allergy. If you have been presented with one of these scenarios in the past, consider taking some tests. The standard test for penicillin allergy involves skin-prick testing (introducing the allergic substance/allergen by scratching the skin) and intradermal testing (injecting the allergen just beneath the skin surface). A positive result would confirm you should not take penicillin. In the case of a negative result, this would then be followed by a controlled "provocation" test in a clinical setting with oral doses of penicillin to truly ensure that you are not allergic to the drug. You should wait four weeks after a severe allergic reaction before arranging a test, as the immune response may under-respond to testing. You should also stop taking oral antihistamines for at least one week to reduce the chances of getting a false-negative result. It is known that patients can "lose their allergy over time". Wu states that the risk of penicillin allergy can reduce to less than 15 per cent after 10 years of avoidance. But it is still prudent to be retested before it is deemed safe for future courses of penicillin. Of all the drug allergies, penicillin allergy is the most common. It is encouraging that nowadays I am seeing fewer patients insisting on antibiotics for relatively minor viral infections, which reduces the risk of resistance for themselves. For patients who have been labelled with penicillin allergy, it is worth getting tested, so your antibiotic options are less limited in the future. life@scmp.com Dr Ray S. H. Ng is a member of the Royal College of General Practitioners (Britain). He practises as a family physician in Central. info@medicinemadeclear.com