Drug and antibiotic allergy overview


IMPORTANT The information provided is of a general nature and should not be used as a substitute for professional advice. If you think you may suffer from an allergic or other disease that requires attention, you should discuss it with your family doctor. The content of the information articles and all illustrations on this website remains the intellectual property of Dr Raymond Mullins and cannot be reproduced without written permission.


When evaluating patients with possible drug allergy, one examines the circumstances surrounding the episode, the results of allergy testing, and at times, deliberate challenge. The best time to sort it out is close to the time of the event, when both the story and results of testing are most accurate.

Pain killers and antibiotics are the most common causes of drug allergy

Allergic reactions have been described to a large number of medicines. Reactions to pain killers/arthritis tablets (such as aspirin, ibuprofen, naproxen) and antibiotics are the most common, but reactions have been described to many other medicines, including some herbal remedies such as Echinacea, Royal Jelly and chamomile.

Who is more likely to suffer drug reactions?

Allergic reactions to medicines are no more likely to occur in people with other allergies (such as hay fever, asthma or eczema) than anyone else in the genera population. While there are reports of some families who have many people with allergic reactions to medicine, most drug allergies are not inherited.

Evaluating the person with possible drug allergy

When evaluating patients with possible drug allergy, one examines the:

•History of the episode,

•Results of allergy testing, and at times,

•Deliberate challenge with the medication.

History of the reaction

The timing and the symptoms experienced may help classify the event as unlikely, possibly or probably representing an allergic reaction.

Unlikely: For example, a headache or upset stomach after a medicine might be a side-effect, but is not an allergic reaction.

Possibly: A rash starting a few days after taking a medicine might be due to the medicine, or due to another cause, such as an infection being treated.

Probably: The sudden onset of a generalised rash, difficulty breathing and stomach upset within an hour or so of taking a new medicine suggests a severe allergic reaction known as anaphylaxis.

Drug allergy may be immediate or delayed

Not all drug allergy is of rapid onset with hives and difficulty breathing. Some reactions are still causes by an interaction between the medicine and the immune system, and may only appear after the medicine has been taken for several days, and sometimes even after the medicine has been ceased.

Drug allergy testing

Allergy testing can be done for some but not all medications. Allergy testing has been shown to be useful for assessing possible allergic reactions to some antibiotics, anaesthetic drugs and latex. When considering antibiotic allergy testing, one requires an intravenous solution for accurate testing, and some medicines come only in tablet form. For other medicines, such as painkillers (e.g. aspirin or tablet-only antibiotics), there is no accurate test to confirm the presence of allergy.


There is no test for side-effects!

Antibiotic allergy testing

Unfortunately, allergy testing with antibiotics is not a hundred per cent accurate. This can be due to:

•The absence of availability of intravenous forms of some medicines for testing.

•Many patients allergic to medications are allergic to the breakdown product of the drug, not the native drug itself. It is often difficult (or impossible) to obtain commercial extracts for allergy testing.

•The passage of time. Many patients with antibiotic allergy will grow out of their sensitivity with time, with their skin tests turning negative after a few months. Testing many years down the track may be negative, even if the person was allergic in the first place.

•When rash is the main symptom, it is often difficult to determine whether the cause was the antibiotic, or the infection that prompted its use.

Interpreting the results of drug allergy testing

In interpreting the results of drug allergy tests, a positive result usually indicated that the person is sensitive to that medication. When results are negative, however, the possible explanations can be that:

•The patient is not allergic now and never was, or

•That the patient was allergic and has grown out of their sensitivity, or

•The test result is inaccurate.


When it is important to prove or disprove sensitivity, deliberate challenge is sometimes required to determine the presence of absence of drug allergy. This is normally performed under medical supervision, using small doses first. Before deciding on a challenge, one uses the story to judge the risk of doing so, and if a challenge is undertaken, its location (eg. Outpatient or in hospital)

Risks of challenge vs risk of not challenging

There has been a movement to try to remove unnecessary antibiotic allergy labels where possible. There is potentially a risk of antibiotic challenge at times but also a risk of not challenging. A number of studies have shown that those labeled as being allergic to penicillin needing hospital treatment end up being given more expensive and less effective antibiotics with more side-effects and longer hospital stays and greater risk of developing antibiotic resistance from having to use more broad spectrum drugs.

Desensitisation to medication

Even if a person is allergic to a medication, there are protocols available to temporarily “desensitize” them to the drug to allow its use in important situations, but the allergy returns once the drug is stopped.

Special cases

Sulphonamide antibiotics: there is no blood allergy test available and currently no liquid form for testing available. If allergic, one should avoid all sulphonamide antibiotics, trimethoprim and sulphasalazine (used to treat inflammatory bowel disease). It is uncertain if dapsone should be avoided. One does not need to avoid drugs that are not sulphonamide antibiotics that only contain a sulphur molecules (eg. some pain killers or diabetes medicines).

Penicillin allergy: most allergic to one penicillin are advised to avoid all drugs in this group. The exception may be allergy to flucloxacillin where allergy may be restricted to this drug alone. Most allergic to penicillin do not need to avoid cephalosporins.

Cephalosporin allergy: most allergic to one drug will only be allergic to that drug and maybe a couple of structurally similar medicines, but do not need to avoid all cephalosporins.

Xray contrast reactions: there is no relationship between allergy to iodine-containing xray contrast and allergy to seafood. Even when reactions to xray contrast are of rapid onset, allergy testing is usually negative. There is no test for delayed reactions such as rashes starting hours later or the next day. If allergy testing is needed, one needs to know the exact contrast used so that this drug can be used for testing.

Last reviewed 5 April 2020