Drug allergies


Drug allergies are particular types of adverse reactions that can occur following drug administration.

Drug allergies are more common than one might think and, for this reason, they are considered a real public health problem. A problem that further increases if we consider that, in many cases, we are faced with cross-allergy situations.

Please note

When we talk about drug allergies we refer to the allergic reactions triggered by the active ingredients responsible for the therapeutic action and not to the allergic reactions that can occur due to any excipients contained in the finished medicinal product (eg lactose or gelatin allergy) .

What are drug allergies?

Drug allergies - together with side and secondary effects - belong to the set of adverse reactions that can occur after taking the drugs.

More specifically, drug allergies are included in the group of type B adverse reactions, that is, reactions that are not predictable and do not depend on the amount of drug taken . In fact, in some cases, drug allergies also occur with very small doses.

However, there is talk of drug allergies when the intake of the latter causes an unexpected reaction of the immune system . In other words, drug allergies are a particular condition in which there is an excessive and abnormal response from the immune system that identifies the drug as a foreign agent potentially harmful to the body and attacks it. The reaction of the immune system gives rise to the release of mediators that lead to the onset of clinical manifestations and typical symptoms of allergies.

In this regard, we recall that drug allergies are mostly IgE-mediated (type I hypersensitivity reactions), or cell-mediated (type IV hypersensitivity reactions).


  • IgE-mediated drug allergies : these allergic reactions are mediated by immunoglobulin E and are characterized by the release of histamine, leukotrienes, prostaglandins and other mediators that give rise to an inflammatory response associated with edema, vasodilation and bronchoconstriction. This type of allergic reaction has a rapid onset (in fact, it usually occurs within an hour of taking the drug).
  • Cell-mediated drug allergies : they are allergic reactions mediated by T lymphocytes which, recognizing the drug as foreign, give rise to inflammatory processes . Usually, such reactions are delayed and tend to occur 48-72 hours after taking the drug.


Drug allergies can occur in any individual, regardless of gender and age.

However, allergic reactions to drugs are generally less frequent in the elderly and in children, probably due to the involution of the immune system that occurs in the former and due to the immaturity of the immune defenses in seconds.

Furthermore, it appears that women are more likely than men to experience cutaneous allergic reactions and a greater risk of experiencing anaphylactic shock following parenteral administration of contrast media.

Causes and Risk Factors

What are the causes and risk factors of drug allergies?

As mentioned, drug allergies occur following an abnormal and excessive reaction of the immune system shortly after taking the drug. This reaction is unexpected and unpredictable, in fact, it is not possible to know if an individual is allergic to a certain drug if he does not take it first, except in the case of possible cross-allergic reactions.

Deepening: Cruel Drugs Allergies

We talk about cross drug allergies when the allergic event occurs not only after taking a certain drug, but also after taking drugs with a chemical structure similar to the latter and / or with a mechanism of action similar.

In other words, if an individual is allergic to antibiotics belonging to the penicillin family, there is a high probability that he is also allergic to cephalosporins, another family of antibiotics but with a chemical structure similar to that of penicillins. Non-steroidal anti-inflammatory drugs (NSAIDs) are other drugs that often give rise to cross-allergies.

However, the causes for which the immune system responds so excessively to taking the drug are still not clear.

As for the risk factors, instead, we remember that the appearance of drug allergies can be influenced by:

  • Route of administration : the route of administration of the drug greatly affects the type of allergic reaction that may occur. Indeed, topical administration will favor the appearance of allergic reactions localized at the site of drug administration; while oral and parenteral administrations favor the appearance of systemic allergic reactions . Furthermore, the parenteral route of administration appears to be the most risky due to the appearance of rapid, even very severe, allergic reactions (anaphylactic shock).
  • Age of the patient : as mentioned above, children and the elderly develop drug allergies less frequently.
  • History of previous allergic reactions : patients who have experienced allergic reactions in the past after taking certain drugs are more likely to develop new allergies to other active ingredients.
  • Pathologies : patients suffering from certain pathologies may present a greater risk of developing drug allergies. For example, individuals with hyperuricemia are more likely to experience a rash following intake of amoxicillin and ampicillin.

Drugs most responsible for allergic reactions

Although drug allergies can manifest themselves with any active ingredient, some seem to be more involved in the onset of such reactions. Among these, we recall:

  • Antibiotics, in particular β-lactams such as penicillins and cephalosporins, but also sulfonamides, tetracyclines and macrolides;
  • Acetylsalicylic acid and other NSAIDs;
  • allopurinol;
  • antiarrhythmics;
  • Antipsychotics;
  • Anti-tumor chemotherapy (in particular, taxanes, cisplatin and doxorubicin);
  • Contrast media;
  • antihypertensive;
  • anticonvulsants;
  • Anti-tuberculosis;
  • Muscle relaxants.

Did you know that ...

Antibiotics are responsible for most allergic and pseudo-allergic reactions to drugs.

Clinical symptoms and manifestations

The clinical manifestations of drug allergies can be different from individual to individual, both in terms of type and intensity with which they occur.

However, the following table shows the main clinical manifestations of drug allergies.

Affected organ / tissue / apparatus

Clinical manifestations
Skin and subcutaneous tissues
  • Angioedema
  • Skin rash
  • Urticaria
  • Itching and redness
  • rashes
  • Toxic epidermal necrolysis
  • Stevens-Johnson syndrome
Cardiovascular system
  • Cardiac arrhythmias
  • Hypotension
  • Cardiac arrest
Respiratory system
  • Angioedema of the respiratory tract
  • Dyspnoea
  • tachypnoea
  • bronchospasm
  • Cyanosis
  • Respiratory arrest
Gastrointestinal tract
  • Nausea
  • He retched
  • Diarrhea
Central nervous system
  • Dizziness
  • Convulsions

Some of the aforementioned clinical manifestations are mild and can be resolved by stopping the drug. Other clinical manifestations, on the other hand, turn out to be particularly serious and - if not promptly treated - can lead to tragic implications.

In this regard, we recall the severity of the anaphylactic shock, a serious allergic reaction that can occur when an individual allergic to a certain drug - after initial sensitization - comes into contact with the active ingredient responsible for allergy. If not promptly treated, anaphylactic shock can even lead to death.

Pseudo-allergic reactions

Some drugs can give rise to so-called pseudo-allergic reactions . These reactions have clinical manifestations very similar to those caused by real allergies to drugs, but differ in the fact that there is no involvement of the immune system .

In fact, in pseudo-allergic reactions there is indeed the release of inflammatory mediators, but this release is not caused by the intervention of the immune system.

A classic example of pseudo-allergic reaction is represented by the urticaria induced by the administration of morphine.

Unfortunately, due to the similarity between clinical manifestations, in many cases it is very difficult to distinguish pseudo-allergic reactions from drug allergies.


Drug Allergy Diagnosis Test

Unfortunately, the diagnosis of drug allergies is not as simple as one might think, since often the patient's medical history is not sufficient to determine exactly whether an allergy is present, just as there are no specific and reliable allergy tests for every existing active ingredient. Currently, in fact, the allergological tests available for the detection of drug allergies are few and can be considered reliable only for some active ingredients.

Skin tests

Among the skin tests that can be used to identify possible drug allergies we find the prick test, the patch test and the intradermal reaction. However, remember that the prick test and the intradermal reaction are reliable and effective only for drugs that give rise to real allergies (therefore, reactions mediated by the immune system) and not for drugs that cause pseudo-allergic reactions.

Finally, it should be remembered that skin tests for a given drug should not be done if the patient has experienced serious adverse reactions following therapy with that drug.

Lab test

In the diagnosis of drug allergies, laboratory tests can also be used. In detail, it is possible to use specific IgE dosages . Of course, such a test is only effective for IgE-mediated allergies and generally has limited validity. In fact, specific IgEs tend to decrease and disappear over time, both at the blood level and at the level of the mast cells.

This type of laboratory test is especially useful for determining allergies to drugs such as β-lactam antibiotics and other antibiotics, local and general anesthetics, heparin and other high molecular weight active ingredients.

The dosage of specific IgE, however, is the preferred allergy test in cases where the execution of skin tests is dangerous for the patient's health.

Tolerance test

Tolerance tests involve taking a given drug at increasing doses that are administered at certain time intervals. This type of test is not performed for drugs that certainly cause an allergic reaction to the patient, nor for those that are strongly suspected. Rather, the tolerance test is performed to identify a potential alternative drug with different chemical structure and / or mechanism of action than the drug to which the patient is allergic.

Of course, given the delicacy of the procedure, the tolerance test must always be performed in a hospital setting and under the strict supervision of an allergist doctor .

Please note

All the above tests must be prescribed and performed by doctors experienced in adverse drug reactions.

Treatments and Treatments

Unfortunately, there is no cure that can definitely treat allergies, therefore, the only remedy currently available is prevention . Prevention that must be carried out by avoiding contact or taking the drug that triggered the allergic reaction.

The possible treatment of drug allergies, therefore, is only symptomatic and aims to reduce the excessive immune response and / or limit and eliminate the clinical manifestations and the resulting symptoms.

In milder cases, the clinical manifestations of drug allergies resolve within a few days simply by stopping taking the drug.

In the case in which the clinical manifestations are particularly serious (Stevens-Johnson syndrome, toxic epidermal necrolysis, bronchospasm, cardiac or respiratory arrest, arrhythmias, hypotension, etc.), the intervention of the health personnel is absolutely essential, since the patient will need all the supportive treatments necessary to restore normal health conditions (drug therapies, artificial respiration, etc.).

Please note

In case of allergic reactions, as well as in case of any other unexpected or unexpected reaction after taking a drug, it is necessary to contact the doctor or medical help immediately. In similar situations, moreover, it is always good to bring along the packaging of the medicine and provide the doctor with all useful information (route of administration used, dosage, start of therapy, etc.).

Drug allergies: How to recognize them

Being able to recognize drug allergies can be very important, especially if the resulting events are particularly serious and put the patient's health at serious risk.

Unfortunately, however, it is not always easy to discriminate between allergic reactions and secondary or side effects caused by the drug. In this regard, it should be pointed out that the secondary or collateral effects are undesirable but predictable effects (for example, although not desired, it is well known and predictable that the oral intake of antibiotics may cause alteration of the intestinal bacterial flora with consequent gastrointestinal disorders ).

Drug allergies, instead:

  • They are unpredictable;
  • They occur with clinical manifestations that have nothing to do with the therapeutic action of the drug, but which instead are typical of allergies in general (angioedema, hives and skin rashes of various types, hypotension, bronchospasm, etc.). Moreover, these manifestations tend to disappear in the short term from the interruption of the drug intake and recur after each administration;
  • Generally, they are not established for drugs that have been used and tolerated for years; while the side and secondary effects may occur even if the drug has always been well tolerated;
  • They do not depend on the dose of the drug taken.

However, whether it is drug allergies, whether it is side or secondary effects, in case of any adverse reactions, contact your doctor immediately.


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