Many Australians claim they have an allergy to penicillin when they're admitted to hospital, but researchers say that's often not the case.
Making matters worse, studies also outline that not being able to use the common antibiotics to treat patients tends to lead to worse outcomes.
So how common are the phantom allergies, and what can be done about them? This is what you need to know.
How many people with a penicillin allergy are actually not allergic?
According to a 2015 study, up to 25 per cent of Australian hospital patients say they are allergic to antibiotics, about half of whom say they're allergic to penicillin, or beta-lactam antibiotics.
However, for every 10 patients who say they're allergic to penicillin, research shows nine won't actually be allergic.
"When you go to the trouble of skin testing and challenging people, around 90 per cent don't actually have penicillin allergy," Dr Winnie Tong, an allergy researcher at UNSW and clinical immunologist at St Vincent's Hospital Sydney, said.
Her comments echo the findings of a 2019 Australian study which said more than 90 per cent of allergies may not be legitimate.
"Not only may most penicillin allergies recorded be inaccurate, but many penicillin allergies wane over time," researcher Misha Devchand said at the time.
"Half of people allergic to penicillin will lose their allergy over five years, and 80 per cent over 10 years."
Tong said many of these incorrect allergies originate in childhood.
"Many patients say their parents told them," she said.
"For example, one person has a reaction and the parents are concerned others in the family could be allergic, even though penicillin allergy is not inherited."
How can patients check if they're actually allergic?
Someone who thinks they have a penicillin allergy but hasn't had it clinically confirmed can get it checked by a medical professional, often using skin tests or an oral rechallenge test.
"If you think that you are allergic to penicillin, it may be a good idea to have your allergy reassessed. Maybe you can be 'de-labelled' – that is, having the allergy removed from your health record," Devchand said after her study was released.
However, sometimes it's not a straightforward process. Tong currently has a six- to 12-month waitlist for patients wanting to get their allergy checked, and says there are further barriers to actually getting people to start taking the medicine.
"The problem is you do all this testing and the patient still won't take penicillin," she said.
"Maybe they didn't understand the results, or their GP didn't get the results letter."
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Why are phantom penicillin allergies an issue?
By erroneously saying they can't take penicillin, a patient is opening themselves up to the possibility of worse treatment.
"Antibiotic allergy labels are correlated with increases in length of hospital stay, hospital readmission rates, surgical site infections, and admissions to intensive care units," the 2019 study in Australian Prescriber said.
"Similarly in general practice, penicillin allergy labels are associated with an increased risk of death."
Part of the issue with not being able to take penicillin is it leads to patients being prescribed broader-spectrum or suboptimal medicines as an alternative.
This, in turn, can contribute to antibiotic resistance – the phenomenon in which germs develop the ability to survive treatment from the drugs designed to target them.
"The increased use of broad-spectrum drugs in hospitalised patients with penicillin allergies also contributes to the growing global problem of antimicrobial resistance," Devchand and fellow researcher Jason Trubiano wrote.
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