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Anaphylactic Reactions

By Peter J. Delves, PhD, University College London, London, UK

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Anaphylactic reactions (anaphylaxis) are sudden, widespread, potentially severe and life-threatening allergic reactions.

  • Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling sensations and dizziness.

  • People then rapidly develop severe symptoms, including generalized itching and hives, swelling, wheezing and difficulty breathing, fainting, and/or other allergy symptoms.

  • These reactions can quickly become life threatening.

  • Avoiding the trigger is the best approach.

  • Affected people should always carry antihistamines and a self-injecting syringe of epinephrine.

  • Anaphylactic reactions require emergency treatment.


Anaphylactic reactions are most commonly caused by the following:

  • Drugs (such as penicillin)

  • Insect stings

  • Certain foods (particularly eggs, seafood, and nuts)

  • Latex

But they can be caused by any allergen. Like other allergic reactions, an anaphylactic reaction does not usually occur after the first exposure to an allergen but may occur after a subsequent exposure. However, many people do not recall a first exposure. Any allergen that causes an anaphylactic reaction in a person is likely to cause that reaction with subsequent exposures, unless measures are taken to prevent it.


Anaphylactic reactions typically begin within 15 minutes of exposure to the allergen. Rarely, reactions begin after 1 hour. Symptoms vary, but each person usually has the same symptoms each time.

The heart beats quickly. People may feel uneasy and become agitated. Blood pressure may fall, causing fainting. Other symptoms include tingling (pins-and-needles) sensations, dizziness, itchy and flushed skin, coughing, a runny nose, sneezing, hives, and swelling of tissue under the skin (angioedema). Breathing may become difficult and wheezing may occur because the throat and/or airways constrict or become swollen. People may have nausea, vomiting, abdominal cramps, and diarrhea.

An anaphylactic reaction may progress so rapidly that people collapse, stop breathing, have seizures, and lose consciousness within 1 to 2 minutes. The reaction may be fatal unless emergency treatment is given immediately.

Symptoms may recur 4 to 8 hours after the initial exposure or later.


  • A doctor's evaluation

  • Sometimes blood or urine tests

The diagnosis of anaphylactic reactions is usually obvious based on symptoms. Because symptoms can quickly become life threatening, treatment is begun immediately.

If symptoms are mild, the diagnosis can be confirmed by blood or urine tests, which measure levels of substances produced during allergic reactions. However, these tests are usually unnecessary.


Avoiding the allergen is the best prevention. People who are allergic to certain unavoidable allergens (such as insect stings) may benefit from long-term allergen immunotherapy.

People who have these reactions should always carry a self-injecting syringe of epinephrine and antihistamine pills for prompt treatment. If they encounter a trigger (for example, if they are stung by an insect) or if they start to develop symptoms, they should immediately inject themselves and take the antihistamines. Usually, this treatment stops the reaction, at least temporarily. Nonetheless, after a severe allergic reaction and immediately after injecting themselves, such people should go to the hospital emergency department, where they can be closely monitored and treatment can be adjusted as needed. People should also wear a Medic Alert bracelet with their allergies listed.


  • Epinephrine

  • Sometimes a breathing tube

  • Sometimes fluids given intravenously

  • Antihistamines and other drugs

In emergencies, doctors give epinephrine by injection under the skin, into a muscle, or into a vein. If breathing is severely impaired, a breathing tube may be inserted into the windpipe (trachea) through the person’s mouth or nose (intubation) or through a small incision in the skin over the trachea, and oxygen is given through the breathing tube.

Low blood pressure often returns to normal after epinephrine is given. If it does not, fluids are given intravenously to increase the volume of blood. Sometimes people are also given drugs that cause blood vessels to narrow (vasoconstrictors) and thus help increase blood pressure.

Antihistamines (such as diphenhydramine) and histamine-2 (H2) blockers (such as cimetidine—see Table: Drugs Used to Treat Peptic Ulcer Disease) are given intravenously until symptoms disappear. If needed, beta-agonists that are inhaled (such as albuterol) are given to widen the airways and help with breathing.

A corticosteroid is sometimes given to help prevent symptoms from recurring several hours later.

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* This is the Consumer Version. *