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Entonox (gas and air) during your endoscopy procedure


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Entonox is also known as gas and air. It is used as pain relief, and can be used during your endoscopy procedure.

Entonox is a mix of half oxygen and half nitrous oxide. It is often used to treat pain during childbirth. It is also used by the ambulance service and in hospitals for different procedures and conditions.

The exact way in which Entonox works is not fully understood. It is thought to work in the brain and the spinal cord, stopping the pain from being felt. 

Having Entonox

Entonox is self-administered, which means you give it to yourself. A specially trained nurse or doctor will show you what to do.

You will be asked to choose either a mouth piece, which is placed into your mouth. Or a face mask, which is placed over your nose and mouth.

It is important that you hold the mouthpiece firmly between your teeth or lips, or the mask firmly over your nose and mouth to form a good seal.

You will be asked to breathe deeply, for 1 to 2 minutes, before your endoscopy procedure begins. This will make sure that the painkiller is in your body.

Entonox will only flow when you breathe in. The deeper you breathe, the better your pain relief will be.

Benefits of Entonox

  • Entonox is quick to work. It usually works within 2 minutes. Although you might feel some effect after your first few breaths.
  • It wears off quickly once you stop breathing it in, so you will not feel any after effects.
  • You are in control of how much of this pain relief you have.

Risks of Entonox

There is minimal risk involved with having Entonox. The main concern is that the gas might become trapped in your body, but this is only a problem if you have certain medical conditions that affect your lungs. Tell your nurses if you have any medical conditions, so they can check it is safe for you to have Entonox. They will not give it to you if there is any risk that this might happen.

Side effects of Entonox

There are some possible side effects of Entonox. Most side effects can be helped by making sure you breathe in and out at a controlled rate when using Entonox.

Most people find that if they concentrate on breathing slowly, they quickly become used to any sensations of dizziness, feeling sick or tingling. However, if you decide that Entonox does not suit you, alternative pain relief can be given instead.

Feeling dizzy or feeling sleepy

Entonox can make you feel dizzy, but this usually gets better when you stop breathing it in.

If you keep breathing in Entonox at this point, you might become very sleepy. If you do, you will drop the mouth piece or mask and breathe in normal air, which would wake you up again.

It is important that you do not let anyone hold the mask or mouth piece over your nose and mouth or between your lips for you.

Feeling sick (nausea)

Entonox can sometimes cause you to feel sick , but this settles once you have stopped using it.


Entonox can cause a mild tingling sensation in different areas of the body. For example, in the fingers and around the mouth. This usually quickly improves once you stop breathing in Entonox.

Feeling tired or weak (anaemia)

Using Entonox for longer than 6 to 8 hours might cause anaemia, vitamin deficiencies and reduced bone marrow function (this is rare, and is only a risk to in extreme circumstances). Endoscopy procedures usually only last for about 30 minutes, so it is very unlikely that you would have these problems.

Alternative pain relief

You might want to think about intravenous sedation. If you choose this method, then medicine will be given through a small plastic tube that is put into your vein (an intravenous cannula). The medicines are usually a combination of a painkiller and a sedative which make most people feel sleepy and relaxed. The nurses in the endoscopy unit can give you more information about intravenous sedation.

Preparing to use Entonox

Before you can use Entonox, your nurse will record your blood pressure, pulse and the amount of oxygen in your blood stream using a special machine. Entonox might not be the best choice for you, so they will also ask some questions to make sure it is suitable for you.

Please tell the nurse if you:

  • have recently used Entonox for a procedure
  • currently have, or have recently had, an ear infection or an operation on your ears
  • have, or have recently had, eye surgery
  • have recently had a head injury, and needed to go to hospital
  • have recently been scuba diving
  • are pregnant, or think you might be pregnant
  • have recently had a condition causing breathing problems (such as a chest injury, or a collapsed lung)
  • have lung condition called emphysema
  • have chronic breathing problems, such as chronic bronchitis or asthma
  • have been told that you have a low vitamin B12

You will be asked to complete a health questionnaire on the day of your procedure, or during your pre-assessment. There is also an electronic copy of the health questionnaire which can be emailed to you before your appointment. 

After using Entonox

We recommend that you are taken home by a relative or friend after your procedure. Do not drive until after you have rested. Please talk to your doctor or nurse if you have any questions about your recovery.

When you go home

You do not have to take any special precautions if you have used Entonox for your procedure. The effects will have worn off by the time you leave the hospital.



Resource number: 3874/VER2

Last reviewed: October 2018 | Next review: October 2021

A list of sources is available on request.

Where next?

 Contact us

If you need to change or cancel your appointment, or if you have any questions or concerns, please contact the Endoscopy Unit.

Phone 020 7188 8887 Monday to Saturday 9am to 5pm 

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