Anaphylaxis is a severe, potentially life-threatening allergic reaction that can develop rapidly.


Anaphylaxis is a severe and potentially life-threatening reaction to a trigger such as an allergy.

It's also known as anaphylactic shock.

This page covers:


What to do



Symptoms of anaphylaxis

Anaphylaxis usually develops suddenly and gets worse very quickly.

The symptoms include:

There may also be other allergy symptoms , including an itchy, raised rash (hives) , feeling or being sick, swelling (angioedema) , or stomach pain .

What to do if someone has anaphylaxis

Anaphylaxis is a medical emergency. It can be very serious if not treated quickly.

If someone has symptoms of anaphylaxis, you should:

  1. call for an ambulance immediately – mention that you think the person has anaphylaxis
  2. remove any trigger if possible – for example, carefully remove any wasp or bee sting stuck in the skin
  3. lie the person down flat – unless they're unconscious, pregnant or having breathing difficulties
  4. use an adrenaline auto-injector if the person has one – but make sure you know how to use it correctly first
  5. give another injection after 5-15 minutes if the symptoms don't improve and a second auto-injector is available

If you're having an anaphylactic reaction, you can follow these steps yourself if you feel able to.

Read about how to treat anaphylaxis for more advice about using auto-injectors and correct positioning.

Triggers of anaphylaxis

Anaphylaxis is the result of the immune system – the body's natural defence system – overreacting to a trigger.

This is often something you're allergic to, but isn't always.

Common anaphylaxis triggers include:

In some cases, there's no obvious trigger. This is known as idiopathic anaphylaxis.

Preventing anaphylaxis

If you have a serious allergy or have experienced anaphylaxis before, it's important to try to prevent future episodes.

The following can help reduce your risk:

  • identify any triggers – you may be referred to an allergy clinic for allergy tests to check for anything that could trigger anaphylaxis
  • avoid triggers whenever possible – for example, you should be careful when food shopping or eating out if you have a food allergy
  • carry your adrenaline auto-injector at all times – give yourself an injection whenever you think you may be experiencing anaphylaxis, even if you're not completely sure

Read more about how to prevent anaphylaxis .


The time it takes for symptoms of anaphylaxis to develop depends on the cause.

If it was something you ate, such as peanuts, it can take anything from a few minutes to two hours. If it was something that entered your skin, such as a sting or an injection, it will usually take between five and 30 minutes.

Read more about the causes of anaphylaxis.

Allergic reactions can vary in severity. Sometimes they only involve mild itching or swelling, but in some people they can be severe and life-threatening.

Symptoms of anaphylaxis include:

  • a red, raised, itchy skin rash (hives)
  • swelling of certain body parts, particularly the face (angioedema)
  • swelling in your throat and narrowing of your airways, which can cause breathing difficulties and wheezing
  • nausea
  • vomiting
  • a sudden drop in blood pressure, which can make you feel faint and dizzy
  • feeling like something terrible is going to happen

When to seek medical advice

If someone has the symptoms of anaphylaxis, they need urgent medical help, particularly if you know they have allergies.

You should immediately call for an ambulance.


Anaphylaxis is caused by a problem with the immune system, which is the body's natural defence against illness and infection.

In the case of anaphylaxis, your immune system overreacts to a harmless substance and releases a number of different chemicals, such as histamine, to deal with the mistaken threat.


Some of the more common triggers for anaphylaxis are outlined below.

Insect stings

While any insect has the potential to trigger anaphylaxis, the vast majority of cases are either caused by bee or wasp stings.

It is estimated that around one in 100 people will experience an allergic reaction after a bee or wasp sting, but only a small minority of these people will go on to develop severe anaphylaxis.


Peanuts are the leading cause of food-related anaphylaxis, accounting for more than half of all cases.

Other foods known to trigger anaphylaxis include:

  • various types of nuts, such as walnuts, cashew nuts, almonds, brazil nuts and hazel nuts
  • milk
  • fish
  • seafood
  • eggs
  • some types of fruit, such as bananas, grapes and strawberries


Types of medication known to trigger anaphylaxis in a small amount of people include:

  • antibiotics, particularly penicillin-like antibiotics
  • medications used to put people to sleep before they have surgery (general anaesthetic)
  • [non-steroidal anti-inflammatory drugs (NSAIDs)]

Most people sensitive to these types of medication will usually develop anaphylaxis as soon as they begin treatment, although this is not the case with NSAIDs.

The risks of these types of medication are very small, so in most cases the benefits of treatment outweigh the potential risk. For example, the risk of developing anaphylaxis:

  • after taking a NSAID-type painkiller – around one in 1,480
  • after taking penicillin – around one in 5,000
  • after being given a general anaesthetic – around one in 10,000

Contrast agents

Contrast agents are a group of special dyes used in certain medical tests to help certain areas of your body show up clearer on scans such as X-rays.

For example, a contrast agent injected into a blood vessel will help show up any problems in the vessel, such as a blockage, on the X-ray. This is known as an angiography.

The risk of developing anaphylaxis after being injected with a contrast agent is thought to be less than one in 10,000.


Anaphylaxis is a medical emergency that requires immediate medical assistance and treatment.

What to do

If someone has symptoms of anaphylaxis , you should:

  1. Call for an ambulance immediately – mention that you think the person has anaphylaxis.
  2. Remove any trigger if possible – for example, carefully remove any wasp or bee sting stuck in the skin.
  3. Lie the person down flat – unless they're unconscious, pregnant or having breathing difficulties (see advice about positioning).
  4. Use an adrenaline auto-injector if the person has one – but make sure you know how to use it correctly first.
  5. Give another injection after 5-15 minutes if the symptoms don't improve and a second auto-injector is available.

If you're having an anaphylactic reaction, you can follow these steps yourself if you feel able to.

Adrenaline auto-injectors

People with potentially serious allergies will often be given an adrenaline auto-injector to carry at all times. This can help stop an anaphylactic reaction becoming life threatening.

This should be used as soon as a serious reaction is suspected, either by the person experiencing anaphylaxis or someone helping them.

If you've been given an auto-injector, make sure you're aware how to use it correctly.

There are three main types of adrenaline auto-injector, which are used in slightly different ways.

These are:

  • EpiPen – find out how to use an EpiPen
  • Jext – find out how to use Jext
  • Emerade – find out how to use Emerade

Instructions are also included on the side of each injector if you forget how to use it or someone else needs to give you the injection.

Positioning and resuscitation

Someone experiencing anaphylaxis should be placed in a comfortable position.

  • Most people should lie flat .
  • Pregnant women should lie on their left side to avoid putting too much pressure on the large vein that leads to the heart.
  • People having trouble breathing should sit up to help make breathing easier.
  • People who are unconscious should be placed in the recovery position to ensure the airway remains open and clear – place them on their side, making sure they're supported by one leg and one arm, and open their airway by lifting their chin.
  • Avoid a sudden change to an upright posture such as standing or sitting up – this can cause a dangerous fall in blood pressure.

If the person's breathing or heart stops, cardiopulmonary resuscitation (CPR) should be performed immediately.

In hospital

You will need to go to hospital for observation – usually for 6-12 hours – as the symptoms can occasionally return during this period.

While in hospital:

  • an oxygen mask may be used to help breathing
  • fluids may be given directly into a vein to help increase blood pressure
  • additional medications such as antihistamines and steroids may be used to help relieve symptoms
  • blood tests may be carried out to confirm anaphylaxis

You should be able to go home when the symptoms are under control and it's thought they won't return quickly. This will usually be after a few hours, but may be longer if the reaction was severe.

You may be asked to take antihistamine and steroid tablets for a few days after leaving hospital to help stop your symptoms returning.

You will also probably be asked to attend a follow-up appointment with an allergy specialist so you can be given advice about how you can avoid further episodes of anaphylaxis .

An adrenaline auto-injector may be provided for emergency use between leaving hospital and attending the follow-up appointment.


If you have a serious allergy or have experienced anaphylaxis, it's important to try to prevent future episodes.

The main things you can do to reduce your risk are outlined below.

Identify triggers

Finding out if you're allergic to anything that could trigger anaphylaxis can help you avoid these triggers in the future.

If you've had anaphylaxis and haven't already been diagnosed with an allergy , you should be referred to an allergy clinic for tests to identify any triggers.

The most commonly used tests are:

  • a skin prick test – your skin is pricked with a tiny amount of a suspected allergen to see if it reacts
  • a blood test – a sample of your blood is taken to test its reaction to a suspected allergen

Read more about diagnosing allergies and allergy testing.

Avoid triggers

If a trigger has been identified, you'll need to take steps to avoid it in the future whenever possible. See below for advice about avoiding some specific triggers.


You can reduce the chances of being exposed to a food allergen by:

  • checking food labels and ingredients
  • letting staff at a restaurant know what you're allergic to so it's not included in your meal
  • remembering some types of food may contain small traces of potential allergens – for example, some sauces contain wheat and peanuts

Read about living with a food allergy for more information.

Insect stings

You can reduce your risk of being stung by an insect by taking basic precautions, such as:

  • moving away from wasps, hornets or bees slowly without panicking – don't wave your arms around or swat at them
  • using an insect repellent if you spend time outdoors, particularly in the summer
  • being careful drinking out of cans when there are insects around – insects may fly or crawl inside the can and sting you in the mouth when you take a drink
  • not walking around outside with bare feet

Some specialist allergy centres can also offer special treatment to help desensitise you to insect stings (immunotherapy).

Read more about preventing insect stings .


If you're allergic to certain types of medicines, there are normally alternatives that can be safely used.

For example, if you're allergic to:

Always tell any healthcare professional about medicine allergies you have, as they may not be aware of them.

Carry adrenaline auto-injectors

You may be prescribed an adrenaline auto-injector if there's an ongoing risk you could develop anaphylaxis.

There are three types of auto-injector – EpiPen, Jext and Emerade – that are each slightly different. Click on the links for advice about how to use these injectors.

It's important to remember the following:

  • carry an auto-injector at all times – there should be no exceptions; you may also be advised to get an emergency card or bracelet with full details of your allergy and doctor's contact details to alert others
  • extremes of heat can make adrenaline less effective – so don't leave your auto-injector in the fridge or your car's glove compartment, for example
  • check the expiry date regularly – an out-of-date injector will offer limited protection
  • manufacturers offer a reminder service , where you can be contacted near the expiry date – check the information leaflet that comes with your medicine for more information
  • don't delay injecting yourself if you think you may be experiencing anaphylaxis , even if your initial symptoms are mild – it's better to use adrenaline early and then find out it was a false alarm than delay treatment until you're sure you're experiencing severe anaphylaxis

If your child has an auto-injector, they will need to change over to an adult dose once they reach 30kg (approximately 4.5 stone).

Content supplied by NHS Choices