1. Ada
  2. Conditions
  3. Anaphylaxis


Written by Ada’s Medical Knowledge Team

Updated on

What is anaphylaxis?

Anaphylaxis, also known as an anaphylactic reaction, is an extreme allergic reaction affecting multiple groups of organs of the body. Anaphylaxis is a medical emergency and requires immediate medical treatment.

It is characterized by rapidly-developing, life-threatening problems affecting one or several of the following:

  • The airway
  • Breathing
  • Blood circulation

Symptoms usually begin within minutes of exposure to a trigger substance, but may less commonly occur only after several hours and tend to worsen quickly. Typical symptoms include an itchy red rash, coughing or wheezing, difficulty breathing and, eventually, fainting.

Triggers for anaphylaxis are different from person to person, but are often everyday things such as insect stings or certain foods, such as nuts, shellfish or eggs. It does not occur on the first exposure to an allergen, but on subsequent exposures.

People at high risk are recommended to carry around an emergency anaphylaxis kit with an adrenaline injection pen, especially when traveling to areas where medical help is not readily available. If treated quickly and appropriately, most people recover well after an episode of anaphylaxis.

Anaphylactic reactions tend to be more common in children and teenagers, though adults can also have reactions when they are exposed to triggers. The onset of an anaphylactic allergy can occur at any age.


The symptoms of anaphylaxis can come on, progress and become life-threatening very quickly.

The most common early symptoms of anaphylaxis include: 1 2

  • Itchy skin, sometimes developing into hives – a rash of raised, pink or red areas known as wheals
  • General reddening of the skin
  • Wheezing, hoarseness and a tight chest, leading to difficulty breathing
  • Swelling of the lips, tongue and throat, known as angioedema
  • Irritation and inflammation of the eyes and nasal passages
  • Swelling of the eyes, hands and feet
  • Dizziness
  • Nausea and vomiting, especially if the trigger was eaten

As anaphylaxis rapidly progresses to its more severe form, anaphylactic shock, other symptoms may include: 1 3

  • A sense of impending doom
  • A fast or thumping heart
  • Stomach cramps, nausea and vomiting

Advanced symptoms of anaphylaxis include: 1 3

  • A high-pitched wheezing sound, known as stridor
  • Fast, shallow breathing or complete inability to breathe
  • Bluish skin on the lips and tongue
  • Confusion, agitation and anxiety
  • A sudden feeling of weakness or dizziness
  • Loss of consciousness

Types of anaphylactic reaction

Anaphylaxis may be divided into two main types: 1

  • Uniphasic reactions have symptoms that get rapidly worse, but once treated, the symptoms go and then do not return.
  • Biphasic reactions may be mild or severe to start with and then appear to disappear before symptoms increasingly get worse.

Good to know: Rarely, anaphylactic reactions may last for hours or even days before fully resolving.

Causes of Anaphylaxis

Anaphylaxis is an extreme allergic response. Allergies occur when the immune system overreacts to a substance that it wrongly perceives as a threat. These substances are called allergens or triggers. Some people are sensitive to one or more allergens.

Good to know: Even a very small amount of an allergen can trigger an allergic reaction. 4

In the case of anaphylaxis, the body’s allergic reaction goes into overdrive. White blood cells – usually involved in helping the body combat infection – release chemical substances, such as histamine, that lead to strong inflammation-like reactions in the parts of the body affected.

This sets off a series of events which cause the body to experience problems with and potentially even lose control of its vital functions, the respiratory and/or cardiovascular system. The blood vessels widen, which leads to a drop in blood pressure. The tissues in the airways swell, which causes difficulty breathing and may completely block the airways. If air cannot reach the lungs, the blood cannot get essential oxygen. Oxygen-rich blood is needed so that all cells and organs, such as the brain and heart, can work properly. The heart muscle needs oxygen so that it can pump blood around the body. Once one major organ system starts to falter, the others become strained, potentially leading to failure of multiple organs. Organ failure may be life-threatening. 3

Common triggers

The trigger for anaphylactic reactions vary from person to person. Some of the most common triggers include: 5 6

  • Certain foods, including nuts, shellfish, eggs, some fruits
  • Insect stings, particularly wasp stings and bee stings
  • Natural latex, also known as rubber, often found in condoms and cleaning gloves
  • Certain medications, including some antibiotics such as penicillin and non-steroidal anti-inflammatory drugs (NSAIDs), e.g. aspirin
  • Contrast agents: special dyes used in some medical tests such as CT angiograms
  • General anesthetic

In some cases of anaphylaxis, no specific cause for this severe allergic reaction can be found This is called idiopathic anaphylaxis. 7 8

Risk factors

Some factors that may increase the risk of anaphylaxis include: 9

  • A previous anaphylactic reaction
  • Allergies or asthma
  • Certain other conditions, including mastocytosis, an abnormal accumulation of mast cells, which can release histamine and other substances during allergic reactions

Anaphylaxis Diagnosis

Treatment is not delayed in order to carry out diagnostic tests, since an anaphylactic reaction is an emergency, and anyone with presumed anaphylaxis is treated as such.

After life-saving emergency treatment has been started and symptoms have been brought under control, blood tests can be done to confirm anaphylaxis or rule out other causes of the symptoms. One particularly important test measures mast cell tryptase, a chemical released by the immune system in a severe allergic reaction. This blood test should be done as soon as possible and no later than four hours from the onset of symptoms. 3 10

Good to know: The definition of anaphylaxis used by clinicians varies. Some clinicians take the term to mean only very severe symptoms, whereas others use it to describe cases with less severe symptoms. All definitions require the presence of symptoms involving the airways, breathing or circulation. 11

Treatment of Anaphylaxis

Anaphylaxis is a life-threatening emergency and needs immediate treatment. If it is suspected that a person is experiencing anaphylaxis then an ambulance must be called at once and the person transported to the nearest emergency department. 3

Good to know: Even if symptoms subside, a trip to the emergency room is still required, as sometimes an anaphylactic reaction can be biphasic, meaning symptoms may go away after a short time, but then return and rapidly get worse again after an hour or so.

First aid measures

While waiting for medical help, first aid measures should be taken immediately, including: 3 12

Adrenaline injection

Adrenaline should be injected immediately, whenever possible. Many people who are susceptible to anaphylaxis will carry an adrenaline auto-injector, also known as an EpiPen, adrenaline pen or epinephrine. This is a pre-filled syringe with a needle that can be injected by the person having the anaphylactic reaction or by a bystander who knows how to use it.

If there is no response to the initial injection, it is recommended to give a further adrenaline dose after five minutes, if the available autoinjector pen contains a second dose. 13 Adrenaline is effective very soon after being administered and wears off quite quickly.

Good to know: Unless specifically instructed otherwise by a medical professional, adrenaline should be injected only in the middle of the outer side of the thigh and can be injected through clothing if necessary in an emergency. If adrenaline is accidentally injected into another area, get emergency medical treatment immediately. If injecting a young child who may move during the injection, their leg should be firmly held in place to limit their movement before and during the injection. 14

Patient positioning

A person in anaphylactic shock should be placed in a comfortable position. Depending on the symptoms experienced, different positions will be advised:

  • A person who feels faint, due to low blood pressure, should lie flat with their legs raised. Sitting or standing may cause additional heart problems
  • A person with airway and breathing problems may prefer to sit up, as this makes breathing easier
  • An unconscious person who is still breathing should be placed on their side in the recovery position 15
  • A pregnant person should lie on their left side to avoid caval compression, which is when a large vein that carries oxygen to the heart is squeezed, which is more likely when lying on the back or on the right side and can lead to low blood pressure

Removing the trigger

Removing the trigger for anaphylaxis is not always possible, but may involve, for example:

Removing the stinger from a bee sting. Early removal is more important than how the sting is removed.

Rinsing mouth to remove traces of a food allergen. However, attempts to make a person vomit after an anaphylactic reaction triggered by a food allergy can lead to other severe problems. This is therefore not recommended.

Cardiopulmonary resuscitation (CPR)

If the person is unresponsive and not breathing, CPR should be performed until the emergency services arrive. CPR is unlikely to make a person’s heart start beating again if it has stopped. However, it will pump some blood around the body, supplying organs and, most importantly, the brain with oxygen.

Good to know: If untrained, a helping person doesn’t need to spend lots of time trying to find a pulse. If a pulse cannot be found quickly and the affected person is unresponsive and not breathing, CPR should be begun without any delay. One can use hands-only CPR if no second helper is in sight or one is not feeling comfortable doing mouth-to-mouth resuscitation. Hands-Only CPR is CPR without mouth-to-mouth ventilation. The important part is keeping the heart artificially pumping by any means necessary. It involves pushing hard, deep and fast in the center of the chest to a regular rhythm, about twice a second. Some doctors suggested doing this to the beat of the popular song Stayin’ Alive, as one thereby mimics the recommended 100 pumps per minute.

Hospital treatment

People with presumed anaphylaxis are treated in the emergency department. The specifics of treatment depend on how well or unwell overall the person is who experiences an anaphylactic reaction.

Treatment may include: 3 16

  • Adrenaline, also known as epinephrine, usually injected into the thigh muscle and sometimes given multiple times or, in rarer cases, via an intravenous drip.
  • Intravenous fluids to raise blood pressure
  • Oxygen and bronchodilators: inhaled medications used in cases of persistent respiratory problems, that help relax the muscles in the lungs to allow increased airflow to the lungs
  • Steroids and antihistamines

In severe cases, a breathing tube and breathing support through a ventilator may be necessary until the immune reaction settles.

Prevention and management

If a person has had an anaphylactic reaction, they are very likely to have another if exposed to the same allergen again. Subsequent episodes of anaphylaxis are often worse than the first one.

There are several ways to prevent and manage the possibility of having another anaphylactic reaction: 17

Avoiding known triggers

It is important to take steps to avoid exposure to triggers. A person who has experienced anaphylaxis will likely be referred to a specialist allergy clinic to identify allergy triggers and for assessment and advice about how to avoid them in the future.

Good to know: Some people who have had an anaphylactic episode in response to common substances or medication choose to wear a piece of jewelry, often a bracelet that has their allergy engraved on it, to direct medical staff in the case of emergencies. 3

Being prepared for future emergencies

An epinephrine autoinjector pen should be carried at all times. They contain a prescribed single or double 13 dose of anaphylaxis medication that is injected into the thigh during an anaphylactic emergency. A doctor will explain to the person affected how to use this. It is also important that other people in close contact, such as family members, also know how to use the autoinjector.

People at high risk of severe reactions should also have an anaphylaxis action plan for their school or workplace, which includes education on the prevention of exposure and how to act in cases of anaphylactic reaction.

Emergency anaphylaxis kits can also be prescribed in some cases. It is recommended that these include: 18

An adrenaline injection, also known as an epinephrine pen, which is the most important. This works rapidly to reverse many severe symptoms caused by anaphylactic shock, most importantly:

  • Low blood pressure
  • Rapid or abnormal heart rate
  • Poor circulation
  • Swelling
  • Asthma

H1 antihistamines, such as loratadine, which are particularly effective for treating skin symptoms and reducing swelling, as a supporting medication to epinephrine. 19

H2 antihistamines, such as ranitidine, which can also help with cardiovascular symptoms alongside other medications. This also reduces stomach acid, which is produced increasingly due to the body’s excess of histamine; therefore H2 antihistamines may also reduce stomach discomfort.

Good to know: A combination of H1 and H2 antihistamine is commonly used, as this has been shown to be more effective than treatment with H1 antihistamine alone. 20

Allergen immunotherapy

For some people, it is possible to undergo allergen immunotherapy, also known as desensitization or hyposensitization therapy, which means the body is very slowly – over months or years – exposed to the trigger responsible for their allergy. This causes the immune system to get used to the trigger gradually, helping to reduce the severity of the allergic reaction in the future. 21

Other names used for anaphylaxis

  • anaphylactic shock
  • anaphylactic reaction


Q: Anaphylaxis vs anaphylactoid reaction – what is the difference?
A: Anaphylaxis is a life-threatening condition triggered by a wide range of allergens and affecting multiple organ systems. It does not occur on the first exposure to an allergen, but on subsequent exposures.

Anaphylactoid reactions – sometimes also called non-immunologic anaphylaxis – have the same symptoms as anaphylactic reactions, but they do not involve or require previous exposure to the allergen. Rather, they occur on first exposure. Anaphylactoid reactions are sometimes seen in certain medical treatments. These may include reactions to NSAIDs, local anesthetics, cancer treatment with monoclonal antibodies and chemotherapy. 22

Q: Allergic reaction vs anaphylaxis – what is the difference?
A: An allergy is an immune response to something that the body mistakenly perceives as a threat, such as pollen or animal dander. In most cases, people with allergies develop mild to moderate symptoms, which develop locally, e.g. as a skin rash or tearing eyes and a runny nose.

Much more rarely, exposure to a triggering allergen can cause an anaphylactic reaction. This happens when a person’s immune system over-releases chemicals, leading to an entire-body reaction as opposed to a local allergy reaction. Anaphylaxis involves the respiratory and/or cardiovascular system, which can be life threatening.

Q: How long does anaphylaxis take?
A: Initial symptoms typically develop rapidly, reaching peak severity within 3 to 30 minutes. Symptoms may disappear after one or two epinephrine injections. Less commonly, they may then return after a period of one to eight hours, which is known as a biphasic response. Rarely, protracted anaphylaxis may occur, in which symptoms persist for days.

Q: How long does recovery after anaphylactic shock take?
A: It may take a few days to fully recover after treatment. Many people will feel quite unwell and drained for up to about a week.

Q: Will Benadryl stop anaphylaxis?
A: No, emergency treatment must be sought immediately as anaphylaxis is life-threatening. Antihistamine pills such as diphenhydramine, brand name Benadryl, are not sufficient to treat anaphylaxis. They can help to relieve allergy symptoms, but work too slowly and ineffectively on life-threatening symptoms to prevent serious complications from anaphylactic shock.