Drug allergy, why does it occur?

Drug allergy is an adverse reaction of the immune system caused by taking a certain drug, this reaction can sometimes be severe. Although any drug can cause allergy, some do it more frequently than others. In this post we provide you with more information about drug allergy and tell you which are the most allergenic drugs.

What is 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 drug reactions. Of these, 15% are drug hypersensitivity reactions or DHRs. DHRs encompass drug reactions that appear allergic. Among these we can differentiate between allergic DHRs, when the responsible for this reaction is the adaptive immune system (the one that acts specifically against an agent, and has memory to attack it more efficiently if another contact with that agent occurs in the future), and non-allergic DHRs.

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

• Immediate DHRs: they usually happen after the first hour when a new drug treatment is administered. Symptoms related to this type of reactions are urticaria, angioedema, rhinitis, conjunctivitis, bronchospasm, abdominal pain, nausea, vomiting, diarrhea, etc.
• Non-immediate DHRs: they can appear at any time after the first hour of administration and usually present with urticaria or exanthema.

The immune system’s main function is to defend us against external agents such as bacteria, viruses and fungi, detecting and eliminating them to prevent them from causing complications. Most allergies occur when the immune system responds to a “false alarm”, i.e. it identifies a drug, a food or a certain pollen as if they were foreign and harmful substances and reacts to eliminate them.

How does an allergic reaction occur?

In the body we have surveillance cells that detect external agents, mainly mast cells, basophils, lymphocytes and eosinophils. These cells detect external agents that may be potentially pathogenic and, once they detect them, they instruct the body to manufacture antibodies against them. In this case, generally, the antibodies involved are immunoglobulin E or IgE. These antibodies that will be produced will be specific for the substance that generated them. This means that the next time the foreign agent enters the body, we will be prepared with all the artillery to kill it in a specific way.

The problem begins when the foreign agent is a drug. In that case the organism will create antibodies and the next time we take it, it will react in a forceful way causing an allergic reaction.

Once there has been a first contact and the person is “sensitized” and has a predisposition to allergy, he/she will present in his/her organism cells called mast cells and basophils that will present IgE on their surface against the drug. When the person takes the drug, it binds to the IgE of these cells and they release substances responsible for the characteristic symptoms of the allergy, the most important of which is “histamine”.

Histamine is one of the main molecules involved in the inflammation process that occurs in an allergic reaction. Normally, histamine is found in small amounts in our body, and performs essential functions such as participating in the contraction of some blood vessels and stimulating the balance between tissues and blood. However, when an allergic reaction occurs, histamine levels rise and allergy symptoms occur.

When does drug allergy occur?

When a drug triggers this process, it is because the immune system recognizes it as a harmful substance, so it generates specific antibodies against it. This may occur the first time a drug is taken, however, in most cases, drug allergy does not manifest itself until a second exposure has occurred. Drug allergy is one of the types of allergy that can result in more severe symptoms.

Drug allergy and drug side effects, how do I tell the difference?

Medications are essential to treat and cure numerous diseases. As we explained in the article Generic drugs: are they as effective as brand-name drugs, there are specific enzymes responsible for metabolizing drugs, which are encoded by certain genes. Hence, the way in which a drug is metabolized can vary in each individual, which can be more or less effective or generate adverse reactions depending on variations in these genes.

One of the most frequent consultations in the areas of allergology has to do with the frequent confusion between the symptoms of drug allergy and the side effects. It is important to know how to differentiate one from the other, since their consequences are also different. Let us look at each in detail.

What are drug side effects?

Adverse drug reactions can be of different types, in fact, allergic reactions to drugs are a type of adverse reaction unrelated to the pharmacological effects of the drug. There are other adverse reactions that can be caused by a drug and that are described in the package insert, generally mild.

Different drugs, depending on their composition and mechanism of action, may have different side effects. They are usually listed in the package insert as frequent, rare or exceptional depending on their frequency in the population.

Examples of adverse effects that are not related to allergic reactions are diarrhea or mouth sores after treatment with antibiotics; constipation, diarrhea or headache in prolonged treatments with omeprazole (medication to treat heartburn or even symptoms of gastroesophageal reflux disease); or stomach pain after prolonged treatment with anti-inflammatory drugs.

Symptoms of drug allergy

Before describing the most common symptoms, it should be noted that they can appear at various times after taking the drug. Thus, they may appear after taking the drug, one hour or several hours later, and even days or weeks after administration. For this reason it is very important that you know at all times which drugs you have taken, and that you learn to identify the most common allergy symptoms:

• Skin rash, hives, itching
• Fever
• Swelling of lips, tongue or face
• Shortness of breath, wheezing
• Runny nose
• Itchy, watery eyes

Antibiotics: this is a heterogeneous group of drugs in which the most common allergy is due to penicillin. In this case, it is also necessary to be careful with other antibiotic derivatives that can cause the same effect. Sulfonamides also cause allergy with some frequency.

Nonsteroidal anti-inflammatory drugs (NSAIDs): this group includes ibuprofen or aspirin, which are generally used for the treatment of pain. They are classified into different groups according to their chemical structure, so if you have a reaction to one NSAID, it is likely that others in the same group will also cause allergy.

Acetylsalicylic acid (Aspirin®): is a very common medication and is one of the drugs that can cause allergy. It is important to look at the composition of the drugs as many include several active ingredients and this active ingredient is often found together with others in various preparations.

Iodinated contrast media: they are used to give more contrast to the organs in special x-rays such as CT (computed tomography). It is important to know if we have a reaction, since they are administered intravenously.

What is anaphylactic shock and how to prevent it?

Anaphylactic shock is a severe, life-threatening anaphylactic (allergic) reaction to a drug. It is characterized by typical allergy symptoms (itching, rash, etc.) along with a severe drop in blood pressure, narrowing of the airways – which prevents breathing – extreme weakness and loss of consciousness.

Signs that may indicate anaphylactic shock include: itching or reddening of the skin, weak and rapid pulse, swelling of the eyelids, lips, face, genitals or any other part of the body, difficulty breathing, weakness, dizziness or loss of consciousness. In case of anaphylactic shock, treatment should be as fast 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 a drug, it is advisable to check and be very careful with the drugs you take, checking the label. Other measures are also important:

• Inform your doctors so that they take it into account before any treatment, even if it is far from the indication of the drug to which you are allergic.
• Wear a badge that identifies you as an allergic person, and a medical alert indicating specifically which drugs you are allergic to, something that is especially useful in case an emergency occurs and you are unconscious.

Allergy Diagnosis, Treatment and Recommendations

Diagnosis is essential for the treatment of a drug allergy. If your doctor thinks you may have one, he or she may order tests and refer you to an allergist to confirm the suspicion. There are different types of tests, although the most common is the skin test.

As for treatment, this consists of:

Treatment of symptoms, in addition to abandoning the medication, includes:

• Antihistamines: Drugs to inhibit or block the effect of histamine, a molecule which, as we have said, is responsible for allergy symptoms.
• Corticosteroids: Drugs that slow down the inflammatory reaction caused by the release of histamine, and stop the bronchoconstrictor effect produced by the allergic reaction that hinders the patient’s breathing.

For cases of very severe reaction in which 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 by generating vasoconstriction, which avoids the drop in blood pressure and prevents bronchospasm. If its administration becomes necessary, it is advisable to take the patient to a medical center for evaluation.

• Use an alternative treatment for the disease that was being treated with the drug that produces the allergy. If it does not exist and it is absolutely necessary, the specialist will evaluate the available options.

When the person knows that he/she has an allergy to a certain drug, he/she usually has medication for the treatment of the symptoms and even pharmacological options for the treatment of a possible anaphylactic shock: this medication is key in case of accidental ingestion or administration of another drug of the drug family, which the patient has not identified.

We hope that this article has helped you to understand why drug allergy occurs and, above all, to be aware of how important it is to know if you suffer from one, which is essential to avoid more serious consequences such as anaphylactic shock.

At Veritas we are committed to proactive health care, so in our blog you will find many more tips to take care of you and your family. In addition, we also offer you genetic tests to know your predisposition to develop certain diseases. Nowadays, taking care of your health means making informed decisions.

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