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What Is A Drug Allergy?

[fa icon="calendar"] Oct 20, 2020 10:00:00 AM / by Calvin Leong


Drug allergy

A drug allergy happens when the body's disease-fighting system (immune system) produces an adverse reaction to a medicine. Drug allergies range from mild to severe.

Some allergic reactions occur one week or more after you are exposed to a medicine (delayed reaction). A sudden (acute), severe allergic reaction that affects multiple areas of the body is called an anaphylactic reaction (anaphylaxis). Anaphylaxis can be life-threatening. All allergic reactions to a medicine require medical evaluation, even if the allergic reaction appears to be mild.

Pharmacist showing a drug box to a old man in a hospital

What are the causes of a drug allergy?

This condition is caused by the immune system wrongly identifying a medicine as being harmful. When this happens, the body releases proteins (antibodies) and other compounds, such as histamine, into the bloodstream. This causes swelling in certain tissues and reduces blood flow to important parts of the body, such as the heart and lungs.

Almost any medicine can cause an allergic reaction. Medicines that commonly cause allergic reactions (are common allergens) include:

  • Penicillin
  • Sulfa medicines (sulfonamides).
  • Medicines that numb certain areas of the body (local anaesthetics).
  • X-ray dyes that contain iodine.

Young black woman covers her nose with tissue

What are the signs or symptoms of a drug allergy?

Common symptoms of a mild allergic reaction include:

  • Nasal congestion.
  • Tingling in the mouth.
  • An itchy, red rash.

Common symptoms of a severe allergic reaction include:

  • Swelling of the eyes, lips, face, or tongue.
  • Swelling of the back of the mouth and the throat.
  • Wheezing
  • A hoarse voice.
  • Itchy, red, swollen areas of skin (hives).
  • Dizziness or light-headedness.
  • Fainting
  • Anxiety or confusion.
  • Abdominal pain.
  • Difficulty breathing, speaking, or swallowing.
  • Chest tightness.
  • Fast or irregular heartbeats (palpitations).
  • Vomiting 
  • Diarrhoea


Laboratory Pipette with Blue Liquid Over Glass Test Tubes.-1

How is drug allergy diagnosed?

This condition is diagnosed based on a physical exam and your history of recent exposure to one or more medicines. You may be referred for follow-up testing by a healthcare provider who specialises in allergies. This testing can confirm the diagnosis of a drug allergy and determine which medicines you are allergic to. Testing may include:

  • Skin tests. These may involve:
    • Injecting a small amount of the possible allergen between layers of your skin (intra-dermal injection).
    • Applying patches to your skin.
  • Blood tests.
  • Drug challenge. For this test, a healthcare provider gives you a small amount of a medicine in gradual doses while watching for an allergic reaction.

If you are unsure of what caused your allergic reaction, your healthcare provider may ask you for:

  • Information about all medicines that you take on a regular basis.
  • The date and time of your reaction.


How is this treated?

There is no cure for allergies. However, an allergic reaction can be treated with:

  • Medicines that help:
  • Respiratory inhalers. These are inhaled medicines that help open (dilate) the airways in your lungs.
  • Injections of medicine that helps to relax the muscles in your airways and tighten your blood vessels (epinephrine).

Severe allergic reactions, such as anaphylaxis, require immediate treatment in a hospital. You may need to be hospitalised for observation. You may also be prescribed rescue medicines, such as epinephrine. Epinephrine comes in many forms, including what is commonly called an auto-injector "pen" (pre-filled automatic epinephrine injection device).

Follow these instructions at home:

If you have a severe allergy

  • Always keep an auto-injector pen or your anaphylaxis kit near you. These can be lifesaving if you have a severe reaction. Use your auto-injector pen or anaphylaxis kit as told by your healthcare provider.
  • Make sure that you, the members of your household, and your employer know:
    • How to use an anaphylaxis kit.
    • How to use an auto-injector pen to give you an epinephrine injection.
  • Replace your epinephrine immediately after you use your auto-injector pen, in case you have another reaction.
  • Wear a medical alert bracelet or necklace that states your drug allergy, if told by your healthcare provider.

General instructions

  • Avoid medicines that you are allergic to.
  • Take over-the-counter and prescription medicines only as instructed by your healthcare provider.
  • If you were given medicines to treat your reaction, do not drive until your healthcare provider approves of it.
  • If you have hives or a rash:
    • Use an over-the-counter antihistamine as told by your healthcare provider.
    • Apply cold, wet cloths (cold compresses) to your skin or take baths or showers in cool water. Avoid hot water.
  • If you had tests done, you are encouraged to get a coy of your test results. Ask your healthcare provider when your results will be ready.
  • Tell any healthcare providers who care for you that you have a drug allergy.
  • Keep all follow-up visits as told by your healthcare provider. This is important.

Contact a healthcare provider if:

  • You suspect you are having a mild allergic reaction. Symptoms of an allergic reaction usually start within 30 minutes after you are exposed to a medicine.
  • You have symptoms that last more than 2 days after your reaction.
  • Your symptoms get worse.
  • You develop new symptoms.


Get help right away if:

  • You need to use epinephrine.
    • An epinephrine injection helps to manage life-threatening allergic reactions, but you still need to go to the emergency room even if epinephrine seems to work. This is important because anaphylaxis may happen again within 72 hours (rebound anaphylaxis).
    • If you used epinephrine to treat anaphylaxis outside of the hospital, you need additional medical care. This may include more doses of epinephrine.
  • You develop symptoms of a severe allergic reaction.

These symptoms may represent a serious problem that is an emergency. Do not wait to see if the symptoms will go away. Use your auto-injector pen or anaphylaxis kit as instructed, and get medical help right away. Call your local emergency services (999 in Malaysia). Do not drive yourself to the hospital.


  • A drug allergy happens when the body's disease-fighting system reacts badly to a medicine.
  • Drug allergies range from mild to severe. In some cases, an allergic reaction may be life-threatening.
  • If you have a severe allergy, always keep an auto-injector pen or your anaphylaxis kit near you.

This information is not intended to replace advice given to you by your healthcare provider. Make sure you discuss any questions you have with your healthcare provider.


Castells M.: Diagnosis and management of anaphylaxis in precision medicine. J Allergy Clin Immunol 2017; 140: pp. 321-333.

Isabwe G.A.C., Garcia Neuer M., de Las Vecillas Sanchez L., Lynch D.M., Marquis K., Castells M.: Hypersensitivity reactions to therapeutic monoclonal antibodies: phenotypes and endotypes. J Allergy Clin Immunol 2018; 142: pp. 159-170.e2.

Macy E., Contreras R.: Health care use and serious infection prevalence associated with penicillin "allergy" in hospitalized patients: a cohort study. J Allergy Clin Immunol 2014; 133: pp. 790-796.

McNeil B.D., Pundir P., Meeker S., et. al.: Identification of a mast-cell-specific receptor crucial for pseudo-allergic drug reactions. Nature 2015; 519: pp. 237-241.


For more information on drug allergy, visit:

Ministry of Health Malaysia – Drug Allergy

Malaysian Society of Allergy and Immunology – Allergy Self Test Questions

World Allergy Organization – Drug Allergy

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Calvin Leong

Written by Calvin Leong

Calvin Leong holds a Master in Medical Education from the University of Dundee, United Kingdom. He is certified in Clinical Wound Care by the ASEAN Wound Care Association. Calvin has 20 years of clinical and lecturing experience focusing on Mentoring in Healthcare, Traumatology and Medical Sciences. Calvin is HRDC certified trainer. He is also a Life Member of The Malaysian Association for the Study of Pain (MASP) and the Malaysian Society of Wound Care Professionals (MSWCP).

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