Genes Matter

Medication allergies, why do they occur?

Medication allergies are an adverse reaction of the immune system caused by taking a specific drug, and sometimes this reaction can be serious. Although any drug can cause an allergy, some do so more frequently than others. In this post, we provide you with more information about allergies to medication and we give you insight about the most allergenic drugs.

What is an allergy and why does it occur?

According to the European Academy of Allergy and Clinical Immunology, more than 7% of the population is affected by adverse reactions to drugs. Of these, 15% are drug hypersensitivity reactions (DHR). Within these, we can differentiate between allergic DHRs, where the adaptive immune system (the system that acts specifically against an agent and has a memory to attack it more effectively if another contact takes place with this agent in the future) is responsible for this reaction, and non-allergic DHRs.

DHRs can be classified into two useful categories for the assessment and treatment of the patient:

  • Immediate DHRs: usually appear an hour after a new drug treatment is administered. The symptoms related to this type of reaction include hives, angioedema, rhinitis, conjunctivitis, bronchospasm, abdominal pain, nausea, vomiting, diarrhoea, etc. 
  • Non-immediate DHRs: may appear at any time after the first hour of administration, often with hives or a skin rash.   

The main role of the immune system is to protect us against external agents like bacteria, viruses and fungi, detecting and eliminating them to prevent them from causing us complications. Most allergies occur when the immune system responds to a “false alarm,” in other words, it identifies a drug, food or specific pollen as if they were foreign and harmful substances and reacts to eliminate them.  

How does an allergic reaction happen?

Our bodies have surveillance cells that detect external agents, which are mainly mast cells, basophils, lymphocytes and eosinophils. These cells detect external agents that may potentially be pathogens and, once they detect them, they give the body instructions to make antibodies against them. Generally, in this case, the antibodies involved are the immunoglobulins E or IgE. The antibodies produced are specific to the substance that has generated them. This means that the next time the external agent enters the body, the response against that substance will be faster and more efficient.

The problem arises when the external agent is a medicine. In this case, the body will create antibodies and the next time we take it, it will react forcefully causing us to have an allergic reaction.

Once there has been a first contact and the person is “sensitised” and has a predisposition to the allergy, cells called mast cells or basophils will appear in the body which will present IgE bound to their surface against the drug. Then, when the person takes the drug, this attaches to the IgE of these cells and they release substances which are responsible for the symptoms that are characteristic of the allergy, among these, the most important is “histamine”. 

Histamine is one of the main molecules involved in the inflammation process which is produced in an allergic reaction. Normally, histamine is found in small quantities in our body, and it performs essential functions such as taking part in the contraction of some blood vessels and stimulating the balance between the tissues and blood. However, when an allergic reaction takes place, the levels of histamine rise and cause the allergy symptoms. 

When does the allergy to medication manifest itself? 

When a medication triggers this process, it is because the immune system recognises it as a harmful substance, so it generates specific antibodies against it. This can happen the first time you take a drug, however, in most cases, the allergy to medication doesn’t manifest itself until there has been a second exposure. An allergy to medication is one of the types of allergies that can cause the most severe symptoms.

Allergy and side effects of medicines, how can I tell the difference?

Medicines are vital in treating and curing numerous diseases. As we explain in the article Generic Medication: is it as effective as brand name medication?, there are specific enzymes that are responsible for metabolising the drugs, which are coded by specific genes. Hence, the way in which a drug is metabolised in each individual may vary, they can be more or less effective, or generate adverse reactions depending on variations in these genes. 

One of the most frequently asked questions in the area of allergology relates to the common confusion between symptoms of an allergy to medication and side effects.  It is important to know how to differentiate one from the other, as their consequences are also different. Let’s look at some of them in more detail.

What are side effects of medicines? 

There are different types of adverse reactions to drugs. In fact, allergic reactions to medication are a type of adverse reaction which is not related to the pharmacological effects of the drug. There are other adverse reactions that can be caused by medication and are described in the package leaflet, generally these are mild. 

Different drugs, depending on their composition and action mechanism, can present different side effects. The package leaflet usually includes the most common, rare or exceptional effects according to their frequency in the population. 

Some examples of adverse effects that are not related to allergic reactions are diarrhoea or cold sores after treatment with antibiotics; constipation, diarrhoea or headache in prolonged treatments with Omeprazole (medication to treat stomach acidity or even the symptoms of Gastroesophageal reflux disease); or stomach ache after a prolonged treatment of anti-inflammatory drugs.

Symptoms of medication allergies 

Before describing the most common symptoms, we should bear in mind that these may appear at different times after taking the medication. They may appear straight after taking the medication, one or several hours later, and even days or weeks after it has been administered. This is why it is important to know at all times what drugs you have taken, and that you learn to identify the most common symptoms of the allergy: 

  • Skin rash, hives, itching
  • Fever
  • Inflammation of the lips, tongue or face
  • Breathlessness, wheezing when you breathe
  • Runny nose
  • Watery and itchy eyes

Which medications produce the most allergies?

Although all medication can cause allergies, some do so more frequently than others: 

  • Antibiotics: this is a varied group of medications where the most common allergy is due to penicillin. In this case, you have to be careful with other antibiotic derivatives that can cause the same effect. Sulphonamides can also cause allergies fairly frequently.
  • Non-steroidal anti-inflammatory drugs (NSAIDs): within this group we find ibuprofen or aspirin, which are generally used to treat pain. They are classified into different groups according to their chemical structure, so, if you have a reaction with an NSAID, it is likely that others from the same group will also cause you an allergic reaction.  
  • Acetylsalicylic acid (Aspirin®): is a very common medicine and is one of the drugs that can cause allergies. It is important to look at the composition of medication, as many include several active ingredients, and this active ingredient is usually found together with others in different formulations.
  • Iodinated contrast media: are used to enhance the contrast of the organs in special X-rays such as CT scans (computerised tomography). It is important to know whether you have a reaction, as it is administered intravenously.

What is anaphylactic shock and how to prevent it? 

Anaphylactic shock is a serious anaphylactic (allergic) reaction to a medication which puts lives at risk. It is characterised by typical allergy symptoms (itching, skin rash, etc.) together with a serious reduction in blood pressure, narrowing of the airways, -which obstructs breathing-, extreme weakness and loss of consciousness. 

The signs that can make us think a person is presenting with anaphylactic shock are: itching or redness of the skin, weak and accelerated pulse, swollen eyelids, lips, face, genitals or any other part of the body; breathing difficulties, weakness, dizziness or loss of consciousness. In the event of anaphylactic shock, treatment should be as quick as possible. 

In most cases, it is not possible to predict a severe allergic reaction, but if you suspect that you may be allergic to any medication, it is worth checking and being very careful with the drugs you take, always checking the label. Additionally, other measures are important: 

  • Inform your doctors so they can take it into account with any treatment, even if this is unrelated to the use of the drug you have an allergy to.
  • Wear a badge that identifies you as someone with an allergy, and a medical alert stating specifically which drugs you are allergic to, particularly useful in the event of an emergency if you are unconscious.

Diagnosis, treatment and recommendations of the allergy

Diagnosis is essential in treating an allergy to medication. If your doctor believes you may suffer from one, they may request tests and refer you to an allergologist to confirm this suspicion. There are different types of test, although the most common is the skin test. 

This treatment consists of: 

  • Treatment of the symptoms, apart from stopping the medication, treatment includes: 
    • Antihistamines: Drugs to inhibit or block the effect of the histamine, a molecule which, as we already mentioned, is responsible for the allergy symptoms.
    • Corticoids: Drugs that curb the inflammatory reaction caused by the release of histamines and curb the bronchoconstriction effect produced by the allergic reaction, which hinders the patient’s breathing.

For extremely severe cases of reaction where several body systems are affected and there is a risk of anaphylactic shock, the specialist usually prescribes an emergency solution: Intramuscular adrenaline. This drug compensates the effects causing vasoconstriction, which prevents the fall in blood pressure and prevents bronchospasms. If it becomes necessary to administer this drug, it is advisable to take the patient to a medical centre to be assessed.

  • Using an alternative treatment for the disease that was being treated with the medication that caused the allergy. If this doesn’t exist and treatment is absolutely necessary, the specialist will assess the options available. 

When someone knows they have an allergy to a specific medication, they usually have medication to treat the symptoms and even pharmacological options for treatment of potential anaphylactic shock: this medication is essential in the event of accidental ingestion or the administering of other drugs from the family of medication which the patient has not identified. 

We hope this article has been useful for you to understand why medication allergies occur and, above all, to be aware of the importance of knowing whether you suffer from any allergy. This is vital to avoid more serious consequences, such as anaphylactic shock.

At Veritas, we are committed to looking after health proactively, which is why you will find many more tips for looking after yourself and your family in our blog. In addition, we also offer genetic tests  so you can find out your predisposition to developing certain diseases. Indeed, nowadays, looking after your health is about making informed decisions. 

Contact us if you still have any questions or if you would like to receive information about our tests.

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Leave a comment


Sam Andrews

17 January, 2023 9:13 am

Oh, hello. I think this article may help my sister-in-law because her left arm feels extremely itchy right after she took her booster dose of the COVID-19 vaccine last weekend. Thanks for informing us about antihistamines and their function in alleviating symptoms related to allergic reactions. Nevertheless, I should probably take her to see a doctor so she can be tested thoroughly.

Eli Richardson

24 March, 2023 8:18 pm

It really helped when you explained the importance of getting diagnosed if you’re allergic to a type of medication. My aunt just got her meds switched up, but she started developing a rash, and her face got swollen, so she stopped taking them today. That’s why I believe she must read your advice right now and get a doctor to check her potential allergy.

Central BioHub GmbH

18 April, 2023 11:30 am

Allergy is an acute allergic reaction that usually starts in few minutes after getting sensitized by allergens. Sneezing, itchy, red, and watery eyes, shortness of breath, coughing, dermatitis, skin rashes, and swollen lips, face, and tongue are a few of the typical allergy symptoms. The severity of allergy symptoms, however, varies greatly from person to person.
Patients with severe allergic disorders have more severe allergy symptoms leading to anaphylaxis. It is potentially life threatening severe allergic reaction charactreised by severe respiratory distress, multiple organ failure, confusion, dizziness, and fainting.

Anaya Ali

29 April, 2023 12:36 pm

I appreciate how informative your explanations are and how much attention you devote to minutiae. I was hoping you’d consider creating a piece titled “Best Medicine for High Blood Pressure.”