Drug Allergy

About Drug Allergy

It is not uncommon for drugs to induce adverse effects. This means that they may cause some alterations, besides the beneficial effect of the drug itself. Sometimes it may be true allergic reactions, but in most cases these are side effects. The drugs most commonly causing allergy are antibiotics and among them the family of penicillins (eg, amoxicillin). Also, aspirin (acetylsalicylic acid) and NSAIDs are a frequent cause.
Among others, drug allergy can cause symptoms such as:

    • urticaria and angioedema (hives and swelling)
    • anaphylaxis (generalized allergic reaction)
    • contact dermatitis (eczema at the site of application of a cream, for example)
    • itchy rashes

To develop a drug allergy, it is usually necessary to have received the drug or a closely related one on a previous occasion. Therefore, a previous good tolerance of a drug does not rule out an allergy.
There are some people who say they are “allergic to all drugs”. This is scientifically impossible. If you have reactions to various drugs, it should be assessed if these are related and whether it is a true allergic reaction or side effects.
Drug allergy testing is one of the most difficult and tedious subjects in allergy diagnosis. There are, with the exception of penicillins, no comercial test solutions in the right dilutions available. That means all test substances need to be prepared by your doctor or the hospital pharmacy. Secondly, after a drug enters the body, it starts changing. If you swallow a tablet, the juices of the stomach can break it down and if it is injected, the kidney or liver can process the drug into a slightly different molecule (“metabolites”). That also makes it difficult to make a test to assess the allergy, since these changes can not always be mimicked outside of the body. Moreover, we do not always now how the body changes the drug or which parts of the drug are causing the allergy.


The first step is to take a precise clinical history. When you suspect you have reacted to a drug it is very important to know exactly which symptoms you had and when they started and exactly when you had taken the drug. Other questions your doctor will address is, whether you were taking other drugs at that time (eg pain killer, antibiotics, blood pressure medicine, but also vitamin pills or complementary medicine) and whether this was the first time you took this particular drug or if you have taken this drug or a similar drug in the past. Other preexisting diseases or concomitant factors may be important.
Skin prick testing (SPT) is normally possible for every drug whether it is injected or taken in the form of a tablet or suppository. The drug is disolved in an adequate diluent – which can be very difficult for tablets -, a drop is placed on your forearm and punctured with a lancet. If after 20 minutes redness and a swelling (like a mosquito bite) develop, it is possible that you are sensitised to the drug. In patients with non-immediate reactions, readings may be done after several hours and after one to several days. However, depending on the drug and dilution, this reaction can sometimes also be seen in people who are not allergic to the drug and has therefore to be interpreted by a doctor experienced in drug allergy!
When intracutaneous or intradermal testing is performed, an extremely small amount of the soluble drug is injected into the skin. This can only be done with drugs that are available for injection and not with tablets, syrups etc. This test, just like the SPT, is also read after 20 minutes and/or after hours to days. Intradermal testing is more sensitive, but less specific as compared to the SPT.
Unlike pollen, house dust mite and animals, very little blood tests are available for drug allergy. The most validated test is the determination of specific immunoglobulin E (IgE) antibodies, which is quite reliable against penicillins and a few other drugs. Other tests, such as the lymphocyte transformation test, basophil activation test or the lymphocyte activation test may be helpful in individual patients, but should be done and interpreted by doctors experienced with drug hypersensitivity.
The most precise way to test if you are drug allergic (also called the golden standard) is to let you take the drug and see what happens. This is called a controlled challenge test.
A disadvantage of this test is that you can develop symptoms, such as you did when you were first taking the drug. Therefore, in this test, you should always start with a small amount and slow increments of the drug and only under strict supervision of your doctor.
Then, should you develop symptoms, these should not be severe and adequate treatment should be started. It can be very important to clarify the hypersensitivity reaction, if the drug you may have reacted to, is necessary for maintaining your health.


Most of the times, when you are allergic to a drug, an alternative drug is available. If not, the doctor may attempt to desensitise you or, in other words, make you tolerant to the drug again. In this desensitization procedure, you start with a minute amount of the drug and take a little more every 30 to 60 minutes until you can tolerate the amount you need to take. This may take one or two days or even more. This desensitisation procedure is, just like the drug provocation test, not without danger since you may develop your allergy symptoms. Therefore, this is normally done in hospital under strict supervision.
Furthermore, the state of tolerance stays as long as you take the drug daily. If you stop taking the drug or forget to take it a few days, you do lose your tolerance to the drug!


Although many people claim to be allergic to drugs, this is not a very common condition. Therefore, it is important to consult your physician if you develop any unusual symptoms which may be related to a drug, in order to find out if it is a possible allergy or if it is a side effect.
If you have been diagnosed of a drug allergy, you should carry with you a detailed report of your physician which states the diagnosis, to which drug/s you are allergic, which drug/s you should avoid and which are the alternatives you may use. In some cases, when a patient is allergic to a very frequently used drug or has experienced a severe reaction, a medical alert should be considered (bracelet, pendant, etc.).