What Is Drug Allergy?

  • Nadza Dzindo BSc Biomolecular Engineering, Technische Universität Darmstadt, Germany
  • Raadhika Agrawal Bachelor of Medicine and Bachelor of Surgery, Kasturba Medical College, Manipal Academy of Higher Education, Manipal, India
  • Humna Maryam Ikram BS, Pharmacology, University of Dundee, Scotland, UK


Any reaction produced by a medication exhibiting clinical traits associated with hypersensitivity is called a drug allergy. It is quite important to gather information about drug sensitivity allergies to prevent accidentally prescribing, distributing, and administering an inappropriate drug to individuals at risk. Drug allergy research strongly relies on a thorough clinical history that must contain information on the rash and onset of symptoms, along with the number of doses of the drug that were taken prior to the reaction.

It is essential to differentiate between drug allergy and drug intolerance or sensitivity. While drug allergies are defined as adverse drug reactions with a known immunological mechanism by the British Society for Allergy and Clinical Immunology (BSACI), drug intolerance or sensitivity refers to an inability to tolerate the side effects of a medication, typically at therapeutic or sub-therapeutic doses.1

Additionally, one should distinguish between side effects caused by a medication and drug allergies. A drug allergy always has a negative effect and always involves the immune system. It only affects certain people. On the other hand, everybody taking a certain medication runs the risk of experiencing a side effect which usually excludes the immunological system. Any medication action, whether beneficial or negative, that is unrelated to the drug's primary function is referred to as an adverse drug reaction.2

Types of drug allergies

According to their onset time, drug allergies can be classified into immediate and non-immediate or delayed reactions. While non-immediate drug hypersensitivity reactions (DHRs) can occur from more than 1 hour to many weeks after evoking treatment, immediate DHRs typically occur between 1-6 hours following drug administration.3

Immediate hypersensitivity reactions

Nearly all kinds of therapeutic medicines, including antibiotics, anticonvulsants, anaesthetics, neuromuscular blocking drugs (NMBD), chemotherapeutic agents, and non-steroidal anti-inflammatory drugs (NSAIDs), have been associated with immediate onset hypersensitivity.4

The majority of immediate allergic drug hypersensitivity reactions (ADHRs), which are also known as type I reactions, take place within the first hour of the administration of a new medication.

  • Anaphylaxis

Anaphylaxis is a severe form of immediate hypersensitivity. Massive histamine release is involved in this potentially fatal reaction, which can cause breathing problems and low blood pressure. Anaphylaxis usually includes multiple target organs, such as the skin, respiratory, gastrointestinal, cardiovascular, and nervous systems. It is manifested through swelling, hives, lowered blood pressure, and in severe cases, shock.

  • Urticaria (hives)

The second most frequent cutaneous manifestation of drug allergy is urticaria (hives). It usually results from penicillins, sulfonamides, and nonsteroidal anti-inflammatory medicines within 24 hours of drug consumption.5 Hives are welt-like, with elevated red spots or bumps on the skin. They are a sort of swelling that appears on the skin's surface and are brought on by an allergic reaction. Chronic hives are welts that last longer than six weeks and frequently come back over the course of months or years.

Delayed hypersensitivity reactions

Delayed drug hypersensitivities are mostly the result of T-cell mediated reactions which vary in severity and clinical diagnoses such as maculopapular exanthema (MPE), fixed drug eruption (FDE), single organ disease (e.g., drug-induced liver injury (DILI) and kidney diseases), drug reaction with eosinophilia and systemic symptoms (DRESS), Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN).

  • Maculopapular rash

A macule is a flat, reddish spot of skin that appears as part of a rash. In a rash, a papule is a raised patch of skin. A rash with both flat and elevated areas is referred to as maculopapular. Depending on the root cause, a maculopapular rash can arise anywhere on the body and can spread to other locations. The rash normally lasts between 2 and 21 days.

  • Fixed drug eruption

A single (or limited number of) well-defined, round or oval, reddish or violaceous patch or plaque that may blister or ulcerate is a sign of a fixed drug eruption. Oral pharmaceuticals are the most prevalent cause of fixed drug eruption, with antimicrobials and nonsteroidal anti-inflammatory drugs (NSAIDs) being the most prominent causes. Topical or intravaginal medication exposures are less common causes. 

  • Stevens-Johnson syndrome (SJS) and Toxic Epidermal Necrolysis (TEN)

The most severe delayed medication responses not only produce rashes, but they can also affect other organs such as the liver, kidneys, lungs, and heart. Blisters are a symptom of dangerous drug reactions called Stevens-Johnson Syndrome and toxic epidermal necrolysis (TEN), in which the surfaces of the eyes, lips, mouth, and genital region may get damaged.

Common allergenic drugs


Penicillin is a widely used antibiotic that treats bacterial illnesses such as pneumonia, scarlet fever, respiratory tract infections, and others. Penicillin and similar antibiotics are among the most commonly used medications that induce drug allergies.

Sulfonamides can cause delayed hypersensitivity reactions such as Stevens-Johnson syndrome. People who develop an allergic reaction to a sulfonamide antibiotic are sometimes labelled as "sulfur allergic" or allergic to sulfur, sulphur, or sulfa. This phrase should not be used since it is unclear and may lead to confusion. Some people mistakenly believe they will be allergic to non-antibiotic sulfonamides, as well as other sulfur-containing medications or sulfite preservatives.

Nonsteroidal anti-inflammatory drugs (NSAIDs)

Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen or naproxen, have the potential to cause allergic responses. It's worth noting that persons who are allergic to aspirin are also likely to be allergic to other NSAIDs.

Other common allergenic drugs

Other medications that most often cause a reaction include:

  • Local anesthetics
  • General anaesthetics
  • Insulin
  • Chemotherapy drugs
  • Acetylsalicylic acid

Risk factors for drug allergies

Age, gender, genetic polymorphisms, certain viral infections ((HIV and herpes viruses), and drug-related characteristics (e.g., frequency of exposure, mode of administration, molecular weight) are all factors that have been linked to an increased chance of developing a drug allergy. Drug allergies are typically observed in young and middle-aged individuals and are more common in women than males.6 A history of other allergies, such as food allergies or hay fever, as well as a personal or family history of medication allergy, can also raise the risk. Oral administration of drugs is less immunogenic than topical and intravenous/intramuscular treatment. 

Symptoms and clinical presentation

In many situations, it might be challenging to identify whether a reaction is brought on by the drug or something else.

The most common symptoms of a drug allergy include:

  • Hives
  • Rash
  • Irritation
  • Itching of the skin
  • Itching of the eyes
  • Swelling in the mouth and throat
  • Sneezing
  • Redness
  • Bleeding
  • Congestion
  • Difficulty in breathing
  • Feeling faint, light-headed or “blacking out”
  • Anxiety


For an appropriate diagnosis of a drug allergy, you should consult with your healthcare professional and let them know about the following:

  • When the symptoms started
  • What they felt like
  • How long they lasted
  • Any other prescriptions you took during that time, including any over-the-counter medications
  • If you have a family history of drug allergies
  • If you have had an allergic reaction to other drugs or allergens in the past

Skin testing

During allergy skin tests, the skin is exposed to suspected allergy-causing substances (allergens) and then examined for symptoms of an allergic reaction. A skin prick test, often known as a puncture or scratch test, looks for rapid allergic reactions to up to 50 different chemicals at the same time.

Blood tests (serology)

Blood testing (in vitro immunoglobulin E antibody tests) can be appropriate for patients who should not or cannot undergo skin tests. These tests are not utilized to diagnose penicillin allergies.

Management and treatment

Topical corticosteroids and oral antihistamines may improve cutaneous symptoms of allergic drug reactions.6

If you have severe hives or hives that last for more than a few days, seek advice from a medical professional.

Treatment of anaphylaxis must begin immediately. If you experience an anaphylactic reaction, you should get an epinephrine (adrenaline) shot right away, and someone else should dial 911 for immediate medical assistance. If neglected, it can be fatal.

As per the NICE Drug Allergy: diagnosis and management clinical guideline7, people should be referred to a specialized drug allergy service if they have:

  • a possible anaphylactic reaction or
  • a severe non-immediate cutaneous reaction, such as Stevens-Johnson Syndrome, toxic epidermal necrolysis, or drug reaction with eosinophilia and systemic symptoms (DRESS).


Precautions to take to reduce the risk of developing drug allergies include:

  • Avoiding known triggers. If it is known which medication has caused the drug allergy, the person should try to avoid that substance.
  • Healthcare professionals should include information about the potential drug allergy in the patient's medical records.
  • Healthcare professionals should give patients written details about their allergies, including information on drugs they should avoid.
  • Before receiving any form of medical treatment (including dental care), it's critical that the patient discloses to their doctor any medication sensitivities they may have. It's also a good idea to carry a card with information about the allergy or wear a medical alert bracelet or pendant.


Any drug, both over-the-counter and prescription medication, can cause drug allergies in some people. Even if a person has used a drug in the past without incident, they can still experience an allergic reaction to it. Some allergic responses might occur shortly after taking the medication and can take a variety of forms, such as a red, itchy rash or swelling of the lips and eyelids. Very rarely can someone experience a severe reaction known as anaphylaxis. Other allergic reactions may not manifest for days or even weeks. However, if a person has already experienced an allergic drug reaction in the past, they can appear sooner. The most effective strategy to treat a drug allergy is to identify and avoid the allergenic drug substance. When available, alternative drugs with unrelated chemical structures should be used.

If you have signs or symptoms of a drug allergy, see your doctor as soon as possible. It is critical to understand and discuss what constitutes an allergic reaction, a common side effect, and what you can tolerate when taking specific medications.


  1. What is the most common drug allergy?

The most frequently reported medication allergy is penicillin.8

  1. How long does it take for a drug allergy to manifest?

It varies. Some people develop an allergy to a drug the first time they take it. Others might not experience a reaction until their second or even multiple exposures.

  1. Can drug allergies be spread to another person?

No, drug allergies aren’t contagious. A drug allergy cannot be transferred to another individual.


  1. Mirakian R, Ewan PW, Durham SR, Youlten LJF, Dugué P, Friedmann PS, et al. BSACI guidelines for the management of drug allergy. Clinical & Experimental Allergy [Internet]. 2009 Jan [cited 2019 Nov 22];39(1):43–61. Available from: https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2222.2008.03155.x
  2. Edwards IR, Aronson JK. Adverse drug reactions: definitions, diagnosis, and management. Lancet (London, England) [Internet]. 2000;356(9237):1255–9. Available from: https://www.ncbi.nlm.nih.gov/pubmed/11072960
  3. Demoly P, Adkinson NF, Brockow K, Castells M, Chiriac AM, Greenberger PA, et al. International Consensus on drug allergy. Allergy. 2014 Feb 21;69(4):420–37.
  4. Stone SF, Phillips EJ, Wiese MD, Heddle RJ, Brown SGA. Immediate-type hypersensitivity drug reactions. British Journal of Clinical Pharmacology [Internet]. 2014 Jun 20;78(1):1–13. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4168374/
  5. Shipley D, Ormerod AD. Drug-induced urticaria. Recognition and treatment. American Journal of Clinical Dermatology [Internet]. 2001;2(3):151–8. Available from: https://pubmed.ncbi.nlm.nih.gov/11705092/ 
  6. Warrington R, Silviu-Dan F. Drug allergy. Allergy, Asthma, and Clinical Immunology : Official Journal of the Canadian Society of Allergy and Clinical Immunology [Internet]. 2011 Nov 10;7(Suppl 1):S10. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3245433/ 
  7. NICE. Overview | Drug allergy: diagnosis and management | Guidance | NICE [Internet]. Nice.org.uk. NICE; 2014. Available from: https://www.nice.org.uk/guidance/cg183
  8. Albin S, Agarwal S. Prevalence and characteristics of reported penicillin allergy in an urban outpatient adult population. Allergy and Asthma Proceedings [Internet]. 2014;35(6):489–94. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4210656/ 
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

Get our health newsletter

Get daily health and wellness advice from our medical team.
Your privacy is important to us. Any information you provide to this website may be placed by us on our servers. If you do not agree do not provide the information.

Nadza Dzindo

BSc Biomolecular Engineering, Technische Universität Darmstadt, Germany

MSc Genetics and Bioengineering, International Burch University, Bosnia and Herzegovina

Nadža is a Biomolecular Engineering graduate with a special interest in red biotechnology and science communication. She has over one year of experience in the healthcare industry working as a Medical Information Associate and communicating accurate medical and scientific information to both members of the public and healthcare professionals. Her goal is to convey accurate, factual, and understandable information to various audiences, whilst further developing her research and writing skills.

my.klarity.health presents all health information in line with our terms and conditions. It is essential to understand that the medical information available on our platform is not intended to substitute the relationship between a patient and their physician or doctor, as well as any medical guidance they offer. Always consult with a healthcare professional before making any decisions based on the information found on our website.
Klarity is a citizen-centric health data management platform that enables citizens to securely access, control and share their own health data. Klarity Health Library aims to provide clear and evidence-based health and wellness related informative articles. 
Klarity / Managed Self Ltd
Alum House
5 Alum Chine Road
Westbourne Bournemouth BH4 8DT
VAT Number: 362 5758 74
Company Number: 10696687

Phone Number:

 +44 20 3239 9818