What is anaphylaxis?

Anaphylaxis is a serious allergic reaction that affects the whole body and can be life-threatening, so it always needs emergency treatment.

Reactions usually begin within minutes and progress quickly, but can sometimes start two to three hours after you’re exposed to the allergen.

If you are diagnosed with a serious allergy and are at higher risk of anaphylaxis, your doctor may prescribe adrenaline auto-injectors to use in an emergency.

Download our anaphylaxis factsheet

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What causes anaphylaxis?

The most common allergens that can cause anaphylaxis are foods such as peanuts, tree nuts, milk, eggs, shellfish, fish and sesame seeds, although many other foods have been known to trigger it. Some people react even to tiny amounts of food.

Other causes include wasp or bee stings, latex (natural rubber latex) and medicines. In some people, exercise can trigger anaphylaxis – either on its own or if you exercise around the same time you’re exposed to other allergens.

Sometimes people have a reaction and the cause can’t be found. This is called ‘idiopathic anaphylaxis’, which means the cause is unknown.


What are the signs and symptoms of anaphylaxis?

Most healthcare professionals consider an allergic reaction to be anaphylaxis when it involves difficulty breathing or affects the heart rhythm or blood pressure. Any one or more of the following symptoms may be present – these are often referred to as the ‘ABC’ symptoms.


In extreme cases there could be a dramatic fall in blood pressure. The person may become weak and floppy and may have a sense of something terrible happening. Any of the ABC symptoms may lead to collapse and unconsciousness and, on rare occasions, can be fatal.


Swelling in the throat, tongue or upper airways (tightening of the throat, hoarse voice, difficulty swallowing).


Sudden onset wheezing, breathing difficulty, noisy breathing.


Dizziness, feeling faint, sudden sleepiness, tiredness, confusion, pale clammy skin, loss of consciousness.



Other symptoms

Other symptoms that might be present include:

  • a red raised rash (known as hives or urticaria) anywhere on the body
  • a tingling or itchy feeling in the mouth
  • swelling of lips, face or eyes
  • stomach pain or vomiting.


These symptoms can also happen on their own. If you don’t have the ABC symptoms, the reaction is likely to be less serious and is not the same as anaphylaxis, but watch carefully in case ABC symptoms develop.

Why does anaphylaxis happen?

An allergic reaction (including anaphylaxis) happens when the body’s immune system wrongly identifies a food or substance as a threat. When this happens, the body releases chemicals such as histamine in response. It is the release of these chemicals that causes the allergic symptoms.

Getting a diagnosis

If you have had an allergic reaction in the past, visit your GP who can refer you to a specialist allergy clinic if needed. They can find a clinic in your area from the British Society for Allergy and Clinical Immunology (BSACI).

It’s important to get a referral even if your symptoms were mild because it can be hard to tell if future allergic reactions could be more serious.

Once you get a referral, the consultant will discuss your medical history and symptoms with you. They might suggest skin prick tests and blood tests, and for food allergies, food challenge tests, to help diagnose the allergy and work out how serious it may be.

What can mean you’re at higher risk?

Some clues that you might be at higher risk are:

  • you have already had a serious reaction, with any of the ‘ABC’ symptoms
  • you have asthma, especially if it is not well controlled
  • you have reacted to a tiny amount of your allergen.


If you have asthma and it is not well controlled, this could make an allergic reaction worse. Make sure you discuss this with your GP or allergy specialist and take any prescribed medicines.

The National Institute for Health and Care Excellence (NICE) have produced guidance on assessment and referral.


advising about adrenaline for allergies

Treating symptoms

If you are at higher risk of anaphylaxis, you may be prescribed adrenaline to use in an emergency.

Adrenaline comes in pre-loaded adrenaline auto-injectors (AAIs) that are designed to be easy to use. Make sure you know how and when to use them. Ask your healthcare professional to show you how to use your specific brand of AAI. You can also find help and training videos on the manufacturer’s website where you can get a free trainer device to practise with

You must carry two AAIs with you at all times, as you may need to use a second one if your symptoms don’t improve after five minutes or get worse.

Adrenaline auto-injectors

The adrenaline auto-injectors prescribed in the UK are:

What can I do to protect myself?

  • If you have asthma as well as allergies, make sure your asthma is well-managed and regularly reviewed.
  • If you have been prescribed adrenaline auto-injectors, make sure you carry them with you at all times and know how to use them.
  • Think ahead. Have an emergency allergy action plan and make sure those around you understand it.

Allergy Action Plans for children

The British Society for Allergy and Clinical Immunology (BSACI) produce template Allergy Action Plans for children. There are three plans available; a generic plan for individuals assessed as not needing an adrenaline auto-injector, and a personal plan for individuals prescribed an EpiPen or Jext pens.

Allergy Action Plan for adults

Anaphylaxis UK has developed a template Allergy Action Plan suitable for adults.

What to do in an emergency

Find out more

Biphasic anaphylaxis

If you have anaphylaxis, you will need to stay in hospital after a serious reaction. This is because a second wave of symptoms can develop – known as a biphasic reaction.

It is not known exactly how many people will have biphasic reactions. Studies suggest symptoms return in between one and 20 out of every 100 people who have had anaphylaxis.

Around half of biphasic reactions happen within six and 12 hours of the first reaction. They are less common with food allergies than other allergies.


Does the risk of anaphylaxis get less over time?

There is no evidence that the risk of anaphylaxis reduces over time, but some people do outgrow their allergies. This is more common in young children, particularly those who are allergic to cow’s milk, egg and wheat.

What is mastocytosis?

In most cases of anaphylaxis there is a trigger, such as a food, drug or insect sting. But anaphylaxis can also happen in people who have a very rare condition called mastocytosis.

Mastocytosis is caused by too many mast cells (a type of blood cell that forms part of the immune system) collecting in the tissues of the body. These are the main cells that release histamine and other chemicals that cause allergic symptoms. If you have mastocytosis, it’s important to get a diagnosis.

Key messages  

  • If you think you might have an allergy, see your GP.
  • If you are prescribed adrenaline auto-injectors, carry two with you at all times.
  • Make sure you know how to use your adrenaline auto-injectors and get a trainer device to practise with.
  • Use your adrenaline auto-injectors as soon as you notice any signs of anaphylaxis – know the ABC symptoms so you can act quickly.
  • Learn what to do in an emergency.

Download our anaphylaxis factsheet

Download the factsheet