Skip to Content

Anaphylactic Reactions



Peter J. Delves

, PhD, University College London, London, UK

Last full review/revision Oct 2020| Content last modified Oct 2020

Anaphylactic reactions 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.
  • Anaphylactic reactions require emergency treatment.
  • Affected people should always carry a self-injecting syringe of epinephrine.

(See also Overview of Allergic Reactions.)

Like other allergic reactions, an anaphylactic reaction does not usually occur after the first exposure to an allergen (the substance that triggers an allergic reaction) but may occur after a person is exposed to the allergen again. 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 when the person is exposed again, unless measures are taken to prevent it.

Causes of Anaphylactic Reactions

Anaphylactic reactions are most commonly caused by the following:

  • Drugs (such as penicillin)
  • Insect stings and animal venoms
  • Certain foods (particularly eggs, seafood, and nuts)
  • Latex

But they can be caused by any allergen.

Anaphylactoid reactions

Anaphylactoid reactions resemble anaphylactic reactions. However, anaphylactoid reactions, unlike anaphylactic reactions, may occur after the first exposure to a substance.

Also, anaphylactoid reactions are not allergic reactions because immunoglobulin E (IgE), the class of antibodies involved in allergic reactions, does not cause them. Rather, the reaction is caused directly by the substance.

The most common triggers of anaphylactoid reactions include

  • Iodine-containing substances that can be seen on x-rays (radiopaque contrast agents)
  • Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs)
  • Opioids
  • Monoclonal antibodies (manufactured antibodies that target and suppress specific parts of the immune system)
  • Exercise

If possible, doctors avoid using radiopaque contrast agents in people who have anaphylactoid reactions to such agents. However, some disorders cannot be diagnosed without contrast agents. In such cases, doctors use contrast agents that are less likely to cause reactions. In addition, drugs that block anaphylactoid reactions, such as prednisone and diphenhydramine, are sometimes given before certain contrast agents are injected.

Symptoms of Anaphylactic Reactions

Anaphylactic reactions typically begin within 15 minutes of exposure to the allergen. Rarely, reactions begin after 1 hour. Symptoms range from mild to severe, 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, and may become dangerously low (shock). Other symptoms include 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 exposure to the allergen or later. Such symptoms are usually milder than they were at first, but they can be more severe or fatal.

Diagnosis of Anaphylactic Reactions

  • A doctor's evaluation
  • Sometimes blood or urine tests

The diagnosis of anaphylactic reactions is usually obvious based on symptoms such as the following:

  • Symptoms of shock (such as low blood pressure, confusion, cold and sweaty skin, and a weak and rapid pulse)
  • Respiratory symptoms (such as difficulty breathing, a gasping sound when breathing in, and wheezing)
  • Two or more other symptoms of possible anaphylaxis (such as angioedema, hives, and nausea or other digestive symptoms)

Because symptoms can quickly become life threatening, treatment is begun immediately, without waiting for tests to be done.

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.

Prevention of Anaphylactic Reactions

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. 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. 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 medical alert bracelet with their allergies listed on it.

Treatment of Anaphylactic Reactions

  • Epinephrine given immediately
  • Sometimes a breathing tube
  • Sometimes fluids given intravenously
  • Antihistamines and other drugs

In emergencies, doctors immediately give epinephrine by injection under the skin, into a muscle, or sometimes into a vein or bone.

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 (if needed) is given through the breathing tube. A second injection of epinephrine may be needed. If the person's blood pressure is very low, epinephrine may be 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 in the blood vessels. 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) are given intravenously until symptoms disappear.

If needed, beta-agonists that are inhaled (such as albuterol) are given to widen the airways, reduce wheezing, and help with breathing.

A corticosteroid is sometimes given to help prevent symptoms from recurring several hours later, although whether this treatment is necessary is unclear.

Drugs Mentioned In This Article

Generic Name Select Brand Names
diphenhydramine No US brand name
epinephrine ADRENALIN
cimetidine TAGAMET
prednisone RAYOS

Copyright © 2022 Merck & Co., Inc., known as MSD outside of the US, Kenilworth, New Jersey, USA. All rights reserved. Merck Manual Disclaimer