Risks and side effects – anaphylaxis (serious allergy)

About this leaflet 

This leaflet is about the risk of having a serious allergic reaction (anaphylaxis) during a general anaesthetic. It explains what might cause an allergic reaction and what can be done about it. 

General anaesthetics are medicines that give a deep sleep-like state. They are essential for some operations and procedures. During a general anaesthetic you are unconscious and feel nothing.

You can read about different types of anaesthetics in our Patient information leaflets and video resources section.

What is anaphylaxis?

Anaphylaxis is a severe allergic reaction. It can happen around the time of a general anaesthetic when the patient is awake or asleep. The reaction is often to other medicines used during surgery. It normally happens very quickly. It can be life threatening but, if it happens, your anaesthetist will treat you quickly.

If you have an allergic reaction: 

• your blood pressure can fall 
• you may have breathing difficulties 
• you might have swelling and skin rashes.

How likely is it to happen? 

Out of every 10,000 people

These numbers come from research studies. You can find out about the research we used in our Anaesthesia and risk evidence table.

What causes anaphylaxis during a general anaesthetic?

Your anaesthetist uses different types of medicines around the time of a general anaesthetic. 

The medicines that usually cause a severe reaction include the following:

• antibiotics: these are used to reduce the chance of infection
• muscle relaxants: these are necessary for certain types of surgery
• other medicines or substances: antiseptic or dyes which can cause a reaction in some people. 

Your anaesthetist can use other anaesthetics and medicines if you are allergic to certain things. 

What if I have an allergic reaction? 

Your anaesthetist monitors you closely when they give you an anaesthetic. They can treat you immediately if it happens. They might give you:

• an injection of adrenaline 
• oxygen through a mask
• fluids or medicines through a cannula (‘drip’) directly into a vein in your hand or arm.

If you have an allergic reaction before the operation has started:

• if your surgery is very urgent, they will treat you so that you can have the operation, but only if it is safe to do so
• your team might decide to postpone your surgery 
• your team might refer you to an allergy clinic or seek advice from an allergy expert before rescheduling your operation.

If you have an allergic reaction after the operation has started:

• your anaesthetist will treat you immediately
• your operation might be stopped. The decision to stop the operation will depend on how severe your reaction is and how urgent your surgery is.

If you have a severe allergic reaction you may have to stay overnight in the hospital. If your allergic reaction is very severe you may be looked after more closely in an intensive care unit (ICU).

You may be asked to do a blood test at the time of the reaction and again at the allergy clinic if you are referred to one. This helps the allergy team determine whether this was an allergic reaction or something else. 

What can I do myself?

• If you have any type of allergy, tell your nurse or anaesthetist. They will ask you about allergies when they assess you for surgery (known as the preoperative assessment). 
• When you go to hospital, you will be asked several times about allergies. This is normal.
• Bring a list of allergies with you to hospital, to any appointments and on the day of the surgery.
• If you think that you had an allergic reaction in the past, try to find out what happened. You can: 
  • ask the team who looked after you at the time
  • ask your GP to find out what happened.
• Let your relatives or carers know if you have any allergies. If you need emergency care or are very unwell, they can tell your team about your allergies.

Allergy tests

It is not routine to have allergy tests before surgery. It is recommended only for people who have had an allergic reaction in the past.

If you have a serious allergic reaction, you should be referred to a specialist clinic to have allergy tests. 

These tests are usually ‘skin tests’. A healthcare professional will put small drops of the medicine on your skin and, after a few minutes, will look for a reaction (usually a small itchy lump).

If you are allergic to a medicine, it is vital that you do not have that medicine again. 

• There might be other medicines that are similar, which you should also avoid.
• You will get a letter from the allergy clinic about this. Bring this letter with you to all your appointments. 
• You may also be advised to wear a warning bracelet at all times in case you need emergency treatment. 

Do allergies run in families?

Allergies do not run in families.

Some conditions that can cause serious reactions do run in families. These are suxamethonium apnoea and malignant hyperthermia. They are not allergies, but it is important to tell your anaesthetist if anybody in your family has these conditions.

More information about suxamethonium apnoea and malignant hyperthermia is available on our website.

Where can I get more information about anaphylaxis?

• Allergy UK.
• Your GP, or your hospital’s preoperative assessment clinic or anaesthetic department.

This leaflet has been produced by Leila Finikarides for the RCoA, in collaboration with patients, anaesthetists and patient representatives of the RCoA.

Disclaimer

We try very hard to keep the information in this leaflet accurate and up-to-date, but we cannot guarantee this. We don’t expect this general information to cover all the questions you might have or to deal with everything that might be important to you. You should discuss your choices and any worries you have with your medical team, using this leaflet as a guide. This leaflet on its own should not be treated as advice. It cannot be used for any commercial or business purpose. For full details, please click here.

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Sixth Edition, November 2024
This leaflet will be reviewed within three years of the date of publication.

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