Drug allergy

Drug allergy appears as skin rashes and can be detected by blood tests and testing the skin.

What is drug allergy?

Drug allergy is an abnormal reaction that is triggered when the body’s immune system reacts to a drug in an undesirable manner. The reaction may occur during or immediately after treatment with a drug.

Allergic reactions are not caused by overdosing.

Why does drug allergy occur?

It is not known why a person becomes allergic to a drug. People who are allergic to other things such as pollen, animal hair, dust mites, or foodstuff are not more likely to develop allergy to drugs than others.

Drug allergy symptoms

Skin rashes, itchy or not, are by far the most common sign of drug allergy. Most often, the rash appears after a couple of days’ treatment but it may also occur after one or two weeks’ of treatment.

Sometimes, drug allergy manifests itself as urticaria with highly itchy and slightly raised patches that come and go. Initially, they may look like mosquito bites. More rarely, swelling may occur - often around the mouth or the eyes. Urticaria and swelling may be the first sign of a more severe allergic reaction.

Fever, joint and muscle pains, and symptoms from other organs may (less often) be caused by drug allergy.

The most severe allergic reaction is anaphylactic (allergic) shock, which is very rare. Anaphylactic shock may occur from a few seconds to a few hours after the administration of a drug. 

The symptoms vary from mild skin symptoms (itching, skin rash) to more severe symptoms (urticaria, swelling, breathing problems, dizziness, violent vomiting). In few cases very severe reactions include fainting, swelling in the throat and severe breathing problems.

How common is drug allergy?

Allergic reactions to drugs appear in 1-2% of all drug treatments. The vast majority of allergic reactions are mild and relatively insignificant to the individual.

Severe allergic reactions to drugs are rare.

Who should be checked for drug allergy?

Drug allergy testing is offered to people with suspected allergic reactions to drugs that may be needed in the future.

Disproving an allergy will benefit the person, but may also show benefit for society and reduce cost. For example, penicillin is a cheaper and better antibiotic, which has fewer side effects and causes less resistant bacteria than many of the alternatives prescribed to people with suspected penicillin allergy.

What happens during the testing?

When investigating suspected drug allergy it is important to determine the symptoms and their temporal relationship with drug intake. Also, other drugs (including food supplements and herbal medicines) that were administered at the same time, are noted.

Information on which other drugs have been tolerated, or not, is also gathered.

Often, a blood sample is taken to check for antibodies against the suspected drug. If the test is negative, it does not rule out allergy entirely. Firstly, in some cases the cause of an allergic reaction is one of the many substances the drug metabolizes into in the body, and not the drug itself.

Secondly, the test result will be negative if the reaction happened many years ago.

In some cases, the doctor will decide that a skin test is required prior to further testing. If the blood sample and skin test results are negative, the risk of an allergic reaction is low but the only way to disprove an allergy is by means of a challenge test.

Challenge test

During a challenge test, the suspected drug is administered as an injection, a mixture, or a tablet to see if an allergic reaction is triggered. Drug allergy is only confirmed in approx. 10% of patients with suspected allergy.

In most cases, a challenge test will include a single dose of the drug - the standard treatment dose - followed by two hours of observation. If the physician assesses that the patient had a relatively severe reaction, the first dose administered during the challenge test may be a diluted version of the drug. If the person is in fact allergic, the diluted version will trigger milder symptoms compared to a full dose. The challenge test will in these cases be continued at 30 to 60 minutes intervals, most often with a total of three doses, each 10 times more potent than the previous dose, up to the standard treatment dose.

If allergic symptoms occur, the challenge test is stopped and the symptoms treated.

Regardless of the number of doses, the patient will be required to remain at the outpatient clinic for observation for two hours after the final dose. If the patient has had no reaction after two hours, he or she will be required to continue the treatment at home for another 3-10 days to check for a delayed reaction. Delayed reactions usually only cause skin symptoms, such as itching or a rash.

Termination of medication prior to challenge test

You are not allowed to take antihistamines for five days prior to testing, as doing so may mask potential reactions.


For safety reasons, challenge tests are not offered to pregnant women. For practical reasons, underage children are not allowed during challenge test unless in the company of another adult.

Are challenge tests dangerous?

During a challenge test, there is always a small risk of an allergic reaction. Hence, challenge tests are performed with great care and under observation by competent staff.

In some cases, a drip will be inserted in the arm prior to the challenge test. This may be used for the challenge test itself and for emergency treatment should a reaction occur.

Every day, the Clinic performs 10-15 drug challenge tests as described above. Hence, we have large experience with challenge tests and we have never experienced an allergic reaction that was not immediately treatable.


If you become ill or cannot come to your challenge test for another reason, please contact us, as soon as possible, as your time slot will then be used for another patient.

If you have been ill recently, are being treated with antibiotics, or are feeling ill on the day of challenge, please contact us to discuss whether the challenge test should be carried out as planned or be postponed.

Where can I learn more?