About anaphylaxis

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

It is also known as anaphylactic shock.

Signs of anaphylaxis include:

  • itchy skin or a raised, red skin rash
  • swollen eyes, lips, hands and feet
  • feeling lightheaded or faint
  • swelling of the mouth, throat or tongue, which can cause breathing and swallowing difficulties
  • wheezing
  • abdominal pain, nausea and vomiting
  • collapse and unconsciousness

What to do

Anaphylaxis should always be treated as a medical emergency. If available, an injection of a medicine called adrenaline should be given as soon as possible.

Some people with a previous history of anaphylaxis will have an auto-injector of adrenaline.

This should be injected into their outer thigh muscle and held in place for 5 to 10 seconds. Instructions for how to use these auto-injectors can be found on the side of each device.

You should call 999 for an ambulance whether adrenaline has been given or not.

If after 5 to 10 minutes the person still feels unwell, a second injection should be given. This should be given in the opposite thigh.

A second dose may also be needed if the person improves and then becomes unwell again.

The person should lie flat, with their legs raised on a chair or a low table. If they are having difficulty breathing, they should sit up to make breathing easier.

If the person is unconscious, you should move them to the recovery position – on their side, supported by one leg and one arm, with the head tilted back and the chin lifted. If the person's breathing or heart stops, cardiopulmonary resuscitation (CPR) should be performed.

Further treatment will be carried out in hospital.

Read more about treating anaphylaxis.

Causes and triggers

Anaphylaxis is the result of your body's immune system overreacting to a harmless substance, such as food. Substances that trigger allergic reactions are known as allergens.

Anaphylaxis usually develops within minutes of contact with an allergen, but sometimes the reaction can happen up to 4 hours later.

The most widely reported triggers of anaphylaxis are:

  • insect stings – particularly wasp and bee stings
  • peanuts and tree nuts
  • other types of foods – such as milk and seafood
  • certain medicines – such as antibiotics

Read more about the causes of anaphylaxis.

Preventing further episodes

If you know what has triggered anaphylaxis, it's important to take steps to avoid exposure to similar triggers.

You should be referred to a specialist allergy clinic to either find out your allergy triggers or, if you already know what causes it, for further assessment and advice about how to avoid allergens in the future.

You may be given two adrenaline auto-injectors to use during any future episodes of anaphylaxis.

Read more about preventing anaphylaxis.

Who is affected?

Anaphylaxis is not common, but people of all ages can be affected. People with other allergic conditions, such as asthma or the allergic skin condition atopic eczema, are most at risk of developing anaphylaxis.

Although the condition is life threatening, deaths are rare. There are around 20 deaths in the UK each year. With prompt and proper treatment, most people make a full recovery. 

Causes of anaphylaxis

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 shown below.

Insect stings

Most cases of anaphylaxis are caused by wasp and bee stings, although potentially any insect bite or sting can cause anaphylaxis.

It's estimated around 1 in 100 people will experience an allergic reaction after a wasp or bee sting, but only a small number of these people will go on to develop severe anaphylaxis.


More than half of all cases of food-related anaphylaxis are caused by peanuts.

Other foods known to trigger anaphylaxis include:

  • nuts – such as walnuts, cashew nuts, almonds, brazil nuts and hazelnuts
  • milk
  • fish and shellfish
  • eggs
  • some types of fruit – such as bananas, kiwi fruit, grapes and strawberries


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

People sensitive to these types of medicines will usually develop anaphylaxis as soon as they begin a course of treatment, although they may have safely received them in the past.

The risk of anaphylaxis using these types of medicines is very small, so in most cases the benefits of treatment outweigh the potential risk.

For example, the risk of developing anaphylaxis is around:

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

Contrast agents

Contrast agents are a group of special dyes used in some medical tests to help certain areas of your body show up better 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 angiography.

The risk of developing anaphylaxis after being injected with a contrast agent is thought to be less than 1 in 10,000.

Rubber latex

Less than 1 in 100 people in the population has a natural rubber latex allergy. Healthcare, hair, beauty, catering and motor industry workers are more likely to have a latex allergy.

Those with a history of hayfever, asthma, eczema, and certain medical conditions, like spina bifida, are more likely to be affected.

Idiopathic anaphylaxis

Sometimes, despite extensive testing, no trigger can be found for anaphylaxis, and the cause remains unknown. This is known as idiopathic anaphylaxis.

Treating anaphylaxis

If you think somebody is experiencing symptoms of anaphylaxis, you should use an adrenaline injector if one is available. Dial 999 immediately afterwards.

Call 999 straight away if an adrenaline injector is not available.

If you can see a potential trigger, such as a wasp or bee sting stuck in their skin, carefully remove it.

Adrenaline injections

Adrenaline causes the blood vessels to become narrower, which raises your blood pressure and reduces swelling. It also causes the airways to open, relieving breathing difficulties.

An adrenaline injection should be given as soon as a serious reaction is suspected.

The signs of suspected anaphylaxis are:

  • problems breathing
  • feeling faint or dizzy
  • loss of consciousness

The injection can be done by the person with anaphylaxis, but sometimes – if it's a young child or someone who is unconscious, for example – another person may need to do it.

Before attempting the injection, make sure you know what to do. You should read all of the instructions carefully when you, or the person you are responsible for, are first prescribed the injector.

After injecting, the syringe should be held in place for 5 to 10 seconds. Injections can be given through clothing.

After injecting the adrenaline, you should immediately dial 999 for an ambulance, even if the person is starting to feel better.

Most people should experience a rapid improvement in symptoms once the adrenaline has been used.

If there's no improvement after 5 to 10 minutes, you should inject a second dose of adrenaline, if one is available. This should be injected into the opposite thigh.

Read Medicines and Healthcare products Regulatory Agency (MHRA) 2014 guidelines on how to use an adrenaline auto-injector (PDF, 188kb).

Positioning and resuscitation

In most cases, the person should lie flat, with their legs raised on a chair or a low table, to help maintain bloodflow to the head and heart.

Pregnant women should lie down on their left side to avoid putting too much pressure on the large vein that leads to the heart.

If the person is conscious but having trouble breathing, they should sit up to make breathing easier.

If the person is unconscious, check that their airways are open and clear, and also check their breathing. Then put them in the recovery position to make sure they don't choke on their vomit.

Place the person on their side, making sure they are supported by one leg and one arm. Open the airway by tilting the head and lifting the chin.

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

Admission to hospital

Even if adrenaline is given, the person will need to go to hospital for observation – usually for 6 to 12 hours – as symptoms can occasionally return during this period.

While in hospital, an oxygen mask can be used to help breathing, and fluids given by an intravenous drip directly into a vein can help increase blood pressure.

As well as adrenaline, additional medications such as antihistamines and corticosteroids can be used to help relieve symptoms. Blood tests may also be carried out while you're in hospital to confirm anaphylaxis.

You should be able to leave hospital when the symptoms are under control and it's thought they will not return quickly. This may be after a few hours, but you may have to stay in hospital for a few days if the symptoms were severe.

You may be asked to take antihistamines and corticosteroid tablets 2 to 3 days after leaving hospital to help stop your symptoms returning.

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

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

Read more about preventing anaphylaxis.

Preventing anaphylaxis

If you have anaphylaxis, you should be offered advice and medication to help prevent further episodes.

Allergy clinic

You should be referred to a specialist allergy clinic for tests to find out what caused the anaphylaxis. Knowing what allergen triggered the allergic reaction can help you avoid further episodes of anaphylaxis.

Some of the tests commonly used to determine allergies include:

  • a skin prick test – your skin is pricked with a tiny amount of a suspected allergen to see if it reacts by becoming red, raised and itchy
  • a blood test – a sample of your blood is taken to test its reaction to a suspected allergen

Read more about diagnosing allergies.

Adrenaline auto-injectors

You may be prescribed an adrenaline auto-injector if you've had a previous episode of anaphylaxis and there's a risk of you having another episode in the future.

There are 3 types of auto-injector:

  • EpiPen
  • Jext 
  • Emerade

Each type is slightly different, and you should make sure you know how to use your auto-injector correctly.

You can also ask for a "trainer" kit so you can practise giving yourself or your child injections.

The following points are important:

  • Carry your auto-injector(s) at all times – there should be no exceptions. It may also be recommended that you 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.
  • Check the expiry date regularly. An out-of-date injector will offer limited protection.
  • The 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.
  • 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).
  • Don't delay injecting yourself if you think you may be experiencing the beginning of 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 are experiencing severe anaphylaxis.

Avoid triggers

If a trigger has been identified as causing your episode of anaphylaxis, you will need to take steps to avoid it in the future.


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

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

See our page on 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

Some specialist allergy centres also offer special treatment to help desensitise you to insect stings if you are at a particularly high risk of a further sting – for example, if you are a beekeeper or gardener.

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:

  • penicillin – you can normally safely take a different group of antibiotics known as macrolides
  • non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen and aspirin – you can normally safely take paracetamol, but read the ingredients of things like colds medicines carefully to make sure they don't contain NSAIDs
  • one type of general anaesthetic – others are available, or it may be possible to perform surgery using a local anaesthetic or an epidural injection
  • angiotensin-converting enzyme (ACE) inhibitors – alternative blood pressure medications, such as calcium channel blockers, can be used

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

Contrast agents

There may be times when it's necessary to use contrast agents – for example, if you had bleeding inside your brain – even if this places you at risk of anaphylaxis.

In such circumstances, you can be given injections of antihistamines and corticosteroids before the contrast agents, which may help prevent symptoms occurring or at least make them less severe.

Last updated:
20 January 2023