- Reference Number: HEY1579/2025
- Departments: Emergency Department, Paediatric Medicine, Paediatrics
- Last Updated: 23 October 2025
Introduction
This leaflet has been produced to give you general information about your procedure. Most of your questions should be answered by this leaflet. It is not intended to replace the discussion between you and your doctor but may act as a starting point for discussion. If after reading it you have any concerns or require further explanation, please discuss this with a member of the healthcare team caring for you.
What is a Penthrox?
Penthrox is a pain-relieving vapour that is inhaled through a custom-built device known as a ‘green whistle’.
It contains a low dose of an anaesthetic medication, called Methoxyflurane but aims to improve pain rather than send people to sleep.
Currently Penthrox is only licensed for use in adults in the UK and is used in children ‘off label’. In other parts of the world (Australia) it has been used safety in children for over 30yrs.
Why use Penthrox?
It is increasingly being used for children and young adults “off licence” in the UK and around the world due to some key benefits:
- Strong pain relief that works very quickly
- Reduces the need for needles, as the medication is inhaled so does not need to be given into a vein to work.
- Short acting pain relief which wears off quickly.
- The patient is in control, so the patient can use as much as is required.
Who can use Penthrox?
Children over 5years old who have pain from an injury can use Penthrox. It should be avoided if:
- Your child has experienced allergic reactions to anaesthetic medications previously.
- There is a family history of malignant hyperthermia (a condition which causes a very high temperature after having an anaesthetic).
- Your child has severe kidney, liver, heart or circulatory problems.
- Your child is taking certain medications already such as strong painkillers or some types of antibiotics, anti-epileptics, antivirals or sedating antihistamines.
What will happen?
Before using Penthrox a doctor or nurse will show you how to use the device correctly.
Your child will hold the inhaler in their teeth and close their lips around it. They will breathe in and out through their mouth. It’s important they breathe out through their mouth and not their nose. This helps keep the medicine from going into the air.
The medicine has a strong fruity taste and smell, which might make them cough, but the coughing sensation should settle after the first few breaths.
The medicine starts to work after 6 to 10 breaths. After that, they can take more if they feel they need it. The inhaler works for about an hour.
Penthrox can also help during painful procedures, like putting on a cast or dressing a burn. For this, your child will breathe in and out the whole time until it’s done, which can last up to 20 minutes.
The pain relief will last for a little while after they stop using Penthrox.
What happens afterwards?
The most likely side effects can include:
- Headache
- Dizziness
- Dry mouth
- Drowsiness
- Sickness
- Tingling skin
- Double vision
- Flushing of the skin or skin irritation.
These usually wear off within a few minutes of stopping use.
If your child gets too sleepy, they will not be able to hold the mouthpiece, which means they can’t breathe in more medicine. The sleepiness should then go away. Please tell a doctor/nurse if they stay sleepy or they don’t wake up when you speak to them.
Serious side effects are very rare but can include allergic reactions or problems with the liver and kidneys. If your child feels unwell after using Penthrox, return to the hospital & tell the staff they have used Penthrox.
Should you require further advice on the issues contained in this leaflet, please do not hesitate to contact the Paediatric Emergency Department on tel: 01482 482114.
General Advice and Consent
Most of your questions should have been answered by this leaflet, but remember that this is only a starting point for discussion with the healthcare team.
Consent to treatment
Before any doctor, nurse or therapist examines or treats your child, they must seek your consent or permission. In order to make a decision, you need to have information from health professionals about the treatment or investigation which is being offered to your child. You should always ask them more questions if you do not understand or if you want more information.
The information you receive should be about your child’s condition, the alternatives available for your child, and whether it carries risks as well as the benefits. What is important is that your consent is genuine or valid. That means:
- you must be able to give your consent
- you must be given enough information to enable you to make a decision
- you must be acting under your own free will and not under the strong influence of another person
Information about your child
We collect and use your child’s information to provide your child with care and treatment. As part of your child’s care, information about your child will be shared between members of a healthcare team, some of whom you may not meet. Your child’s information may also be used to help train staff, to check the quality of our care, to manage and plan the health service, and to help with research. Wherever possible we use anonymous data.
We may pass on relevant information to other health organisations that provide your child with care. All information is treated as strictly confidential and is not given to anyone who does not need it. If you have any concerns please ask your child’s doctor, or the person caring for your child.
Under the General Data Protection Regulation and the Data Protection Act 2018 we are responsible for maintaining the confidentiality of any information we hold about your child. For further information visit the following page: Confidential Information about You.
If you need information about your child’s (or a child you care for) health and wellbeing and their care and treatment in a different format, such as large print, braille or audio, due to disability, impairment or sensory loss, please advise a member of staff and this can be arranged.
