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Allergic Reaction, Severe

What is a severe allergic reaction?

An allergic reaction is your body’s reaction to a substance that is normally harmless. A substance that triggers your allergy is called an allergen. Allergens can include foods, insect stings or bites, medicines, chemicals, pollen, dust, pet dander, smoke, or other things in your environment.

Symptoms of an allergic reaction usually start within minutes to an hour or two after contact with the allergen. Allergic reactions can be mild or more severe. A severe allergic reaction can be life threatening. You may have severe swelling of the lips, tongue, or throat; itching; skin rash; hives; or trouble breathing. Your blood vessels may leak fluid into the area around them. This can make your blood pressure drop suddenly and cause you to go into shock. The medical term for a severe allergic reaction is anaphylaxis.

What can I expect in the hospital?

Several things may be done while you are in the hospital to monitor, test, and treat your condition. They include:


  • You will be checked often by the hospital staff.
  • Your heart rate, blood pressure, and temperature will be checked regularly.
  • Your blood oxygen level will be monitored by a sensor that is attached to your finger or earlobe.


Testing may include:

  • Arterial blood gas (ABG): A blood test to measure the levels of oxygen and carbon dioxide in your blood
  • Blood or skin tests to check for things you are allergic to
  • Blood, urine, or other tests to monitor how well your organs are functioning
  • X-rays: Pictures of the inside of your lungs to check for fluid


The treatment for a severe allergic reaction depends on the cause, your symptoms, how well you respond to treatment, your overall health, and any complications you may have.

  • You will have a small tube (IV catheter) inserted into a vein in your hand or arm. This will allow medicine to be given directly into your blood and to give you fluids, if needed.
  • You may receive oxygen through a small tube placed under your nose or through a mask placed over your face. In very severe cases, you may need a tube put into your windpipe to help you breathe.
  • Your provider may prescribe medicine to:
    • Help relax your airways
    • Reduce swelling in the airways
    • Reduce your body’s response to the allergen

What can I do to help?

  • You will need to tell your healthcare team if you have new or worsening:
    • Trouble breathing
    • Drooling or trouble swallowing
    • Coughing with chest tightness
    • Fainting
    • Itching
    • Lightheadedness
    • Skin rash or hives
  • Ask questions about any medicine, treatment, or information that you do not understand.

How long will I be in the hospital?

How long you stay in the hospital depends on many factors. The average amount of time to stay in the hospital with a severe allergic reaction is 2 to 3 days.

Developed by RelayHealth.
Acute Care Advisor 2015.1 published by RelayHealth.
Last modified: 2014-04-30
Last reviewed: 2014-04-10
This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.
Copyright ©1986-2015 McKesson Corporation and/or one of its subsidiaries. All rights reserved.

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