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.

Triggers

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.

Foods

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

Medication

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), a type of painkiller that includes ibuprofen and aspirin

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 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
• narrowing of the airways which can cause wheezing and breathing difficulties
• abdominal pain, nausea and vomiting
• collapse and unconsciousness

Read more about the symptoms of anaphylaxis.

What to do

Anaphylaxis should always be treated as a medical emergency.

If you suspect that you or somebody else is experiencing symptoms of anaphylaxis, you should immediately dial 999 for an ambulance.

If available, an injection of a medicine called adrenaline should be given if someone is having breathing difficulties, feeling faint, or has lost consciousness due to suspected anaphylaxis.

Some people with a previous history of anaphylaxis will have an auto-injector of adrenaline. This should be injected into their thigh muscle and held in place for 10 seconds. Instructions for how to use these auto-injectors can be found on the side of each device. 

If the person is conscious, you should place them in a position where they are comfortable and able to breathe easily until the ambulance arrives. If they are feeling faint, they should be laid flat with their legs elevated, if possible.

If the person is unconscious, you should place them in 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 and observation 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 provoke allergic reactions are known as allergens.

Anaphylaxis usually develops within minutes of contact with an allergen, though sometimes the reaction can happen hours later.

The most widely reported triggers of anaphylaxis are:

• insect stings, particularly wasp and bee stings
• nuts
• other types of foods, such as milk and seafood
• certain medications, such as some types of antibiotics

Read more about the causes of anaphylaxis.

Preventing further episodes

If you know what has triggered anaphylaxis, it is important to take steps to try to avoid further exposure to similar triggers.

If the cause of the allergic reaction is not known, you should be referred to a specialist allergy clinic where tests can be carried out to help identify possible triggers.

You may be provided with an adrenaline auto-injector to use during any future episodes of anaphylaxis.

Read more about preventing anaphylaxis.

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

Allergy clinic

To discover what caused the anaphylaxis you should be referred to a specialist allergy clinic for tests. Knowing what allergen triggered the allergic reaction may 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 and allergy testing.

Adrenaline auto-injectors

If you have experienced a previous episode of anaphylaxis and there’s a risk of you having another episode in the future, you may be prescribed an adrenaline auto-injector.

There are three types of auto-injector:

• EpiPen
• Jext
• Emerade

Each type is slightly different, so you should make sure you know how to use your device correctly.

A health professional should train you how to use the auto-injector. You can also ask for a “trainer” kit so you can practise giving yourself (or your child) injections.

Instructions about how to use the different auto-injectors will be also provided with the device and can be found on the manufacturers’ websites.

The following points are important.

• Carry your auto-injector at all times – there should be no exceptions. In some cases, it may be recommended that you carry multiple injectors. Check with your GP or the doctor in charge of your care. It may also be recommended you get an emergency card or bracelet with full details of your allergy and contact details of your doctor to alert others.
• Extremes of heat can make adrenaline less effective, so do not leave your auto-injector in places such as your fridge or the glove compartment of your car.
• Check the expiry date regularly. An out-of-date injector will only offer limited protection.
• The manufacturers offer a reminder service where you can be contacted near the date of expiry. Check the information leaflet that comes with your medication for more information.
• If your child has an auto-injector, they will need to change over to an adult dose once they reach 30kg (4.7 stone).
• Do not delay injecting yourself if you think you may be experiencing the beginning of anaphylaxis, even if your initial symptoms are mild. It is better to use adrenaline early and then find out it was a false alarm than delay treatment until you are sure you are experiencing severe anaphylaxis.

Avoid triggers

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

Food

If the trigger was a certain type of food, taking some basic precautions should help you avoid an episode of anaphylaxis in the future. You can reduce the chances of being exposed to a food allergen by:

• checking the labels of foods you eat
• letting staff at a restaurant know what you are 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

See 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:

• if you encounter wasps, hornets or bees, move away from them slowly without panicking – do not wave your arms around or swat at them
• use an insect repellent if you are planning to spend time outdoors, particularly in the summer
• wear a long-sleeved top and long trousers, socks and shoes, and avoid bright colours and strong perfumes or lotions
• keep the windows of your car closed to stop insects getting inside

Some specialist allergy centres also offer special treatment to help desensitise you to insect stings if you are at a particularly high risk (such as being a beekeeper or gardener).

Read more about preventing insect stings.

Medication

If you are allergic to certain types of medication, there are normally alternatives that can be safely used. For example, if you are 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 cold medicines carefully to ensure they do not 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

It is important you tell anybody giving you medical care about any medication allergies you have. It is safer not to assume they already know.

Contrast agents

There may be times when it is 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 symptoms less severe.

The Map of Medicine is used by doctors throughout the NHS to determine the best treatment options for their patients. NHS Choices offers everyone in England exclusive and free access to this cutting-edge internet resource, which lets you see exactly what your doctor sees.

The information in the Map has been approved by the UK’s leading clinical experts, is based on the best available clinical evidence, and is continually updated. To take advantage of this unique resource go to:

Map of Medicine: 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:

• itchy skin or a raised, red skin rash (hives)
• swollen eyes, lips, hands and feet (angioedema)
• feeling lightheaded or faint
• narrowing of the airways which can cause wheezing and breathing difficulties
• abdominal (tummy) pain, nausea and vomiting
• collapse and unconsciousness

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 dial 999 for an ambulance.

If you suspect somebody is experiencing symptoms of anaphylaxis, call 999 immediately for an ambulance and tell the operator you think the person has anaphylaxis.

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

If available, an adrenaline injection should be given as soon as a serious reaction is suspected. This can be done by the person with anaphylaxis if possible, but in some circumstances (such as cases involving a young child or someone who is unconscious) you may need to inject them yourself. 

Before attempting the injection, make sure you know the correct way to use the specific device available.

There are three types of auto-injectors:

• EpiPen
• Jext
• Emerade

These auto-injectors release adrenaline when jabbed or pressed against the outer thigh.

Make sure you do not accidentally inject into a fatty part of their leg, as the adrenaline cannot move through fat. Also do not inject into a vein or artery, as this can cause dangerous side effects. The injector should only be placed firmly into the muscle of the outer thigh.

Carefully reading the manufacturer’s instructions that come with the auto-injector will teach you how to do this. Make sure you read the instructions as soon as you are first prescribed an auto-injector.

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

Read MHRA (2014) guidelines on how to use an adrenaline auto-injector (PDF, 188Kb).

Most people should experience a rapid improvement in symptoms once the adrenaline has been used. If there is no improvement after five minutes, you should inject a second dose of adrenaline, if one is available. This should be injected into the opposite leg.

If the person has asthma and they have a reliever inhaler (usually blue), this should be used if they are wheezing.

Positioning and resuscitation

If the person is unconscious, check their airways are open and clear and also check their breathing. Then put them in the recovery position (see below) to ensure they do not choke on their vomit.

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

If the person is conscious but experiencing trouble breathing, they may prefer to sit up as this will make breathing easier.

If the person is conscious but feels faint, they should lie flat with their legs elevated, if possible. They should not sit or stand as this could potentially lead to a heart attack.

If a woman is pregnant, they should lie down on their left side to avoid putting too much pressure on one of the large veins that leads to the heart.

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 be admitted to hospital for observation (usually for six to 12 hours) as occasionally symptoms can return during this period.

While in hospital, an oxygen mask can be used to help breathing and you may be given fluids via an intravenous drip (directly into a vein) to help increase your 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 are in hospital to confirm a diagnosis of anaphylaxis.

You should be able to leave hospital when the symptoms are under control and it is felt they will not quickly return. In some cases, this may be after a few hours, but there is a chance you will have to stay in hospital for a few days if the symptoms were severe.

You may be asked to take antihistamines and corticosteroid tablets two to three days after leaving hospital to help prevent a return of symptoms.

It is likely you will be asked to attend a follow-up appointment so you can be given advice about how you can avoid further episodes of anaphylaxis. You may also be given an adrenaline auto-injector for emergency use between leaving hospital and attending the follow-up appointment. 

Read more about preventing anaphylaxis.