Penthrox for Pain Relief During Outpatient Hysteroscopy
What is Penthrox?
Penthrox is a pain-relieving vapour that you breathe in through a small handheld device called the ‘Green Whistle’. It contains a medicine called methoxyflurane, which is a type of anaesthetic.
Penthrox helps reduce the amount of pain you feel during a procedure, although it may not remove the pain completely. It is sometimes used off-licence in outpatient hysteroscopy clinics for procedures where moderate to severe pain is expected (pain score greater than 4 out of 10).
How do I use Penthrox?
Penthrox is self-administered, meaning you control when to use it. A nurse or doctor will always supervise you while you use it.
Before your procedure begins, your nurse will show you how to use the device. It is simple to use—place the mouthpiece between your teeth and close your lips around it.
You will usually start to feel the effects after 6 to 10 breaths. Once it begins to work, you can inhale as needed—either continuously or when you feel more pain.
If you need stronger pain relief, you can cover the small hole at the top of the clear chamber with your finger to increase the dose. Pain relief will continue for several minutes after you stop inhaling.
Penthrox has a fruity taste and smell, which you will get used to as you breathe it in. You may feel light-headed or drowsy while using it.
Below is a short video demonstrates how to use Penthrox. Your doctor can also show you the video during your consultation.
When should Penthrox be avoided?
You should not use Penthrox if you:
- Are allergic to, or have reacted badly to, any anaesthetic gases used during general anaesthesia
- Have a personal or family history of malignant hyperthermia (a rare, life-threatening rise in body temperature during or after anaesthesia)
- Have serious kidney or liver problems
- Have reduced or altered consciousness (for example, due to dementia or confusion)
- Have used alcohol or recreational drugs within the last 24 hours
- Have significant heart or circulation problems (such as arrhythmia or heart failure)
- Are pregnant or breastfeeding
- Have shallow or difficult breathing
- Are under 18 years of age
- Are taking strong painkillers or sedatives
- Are taking strong painkillers or sedatives
Note: Do not exceed 6 ml (2 vials) of Penthrox within 24 hours.
Possible side effects
Common side effects
- Headache
- Dizziness or feeling sick (nausea)
- Dry mouth or cough
- Drowsiness
- Tingling skin
- Double vision
- Flushing or mild skin irritation
Rare side effects
- Allergic reaction (including malignant hyperthermia)
- Liver or kidney damage—very unlikely at the low dose used in outpatient procedures
If you experience any worrying or persistent symptoms after using Penthrox, please contact your doctor or NHS 111.
After your appointment
The effects of Penthrox wear off quickly. If you feel well, you will usually be able to go home shortly after your procedure. However, some patients may need a little extra time to recover in our recovery or waiting area, where refreshments will be available.
Do not drive or operate machinery for 24 hours after using Penthrox.
Take-home messages
- Provides effective pain relief during outpatient hysteroscopy
- Inhaled through a handheld device, self-administered by the patient
- Works within minutes
- Allows you to control your own level of pain relief
- Reduces pain particularly in women who have not had vaginal births or who have a narrow cervix
- Causes minimal drowsiness and allows fast recovery
- May cause mild dizziness, nausea, or headache
- Rarely causes euphoria or disorientation
- Safe, effective, and patient-friendly option for pain relief in office-based procedures
Contact information
Gynaecology Outpatients Department
1st Floor, Lift bank B
Chelsea and Westminster Hospital
E: chelwest.oph@nhs.net
T: 07468 740 422 (Mon–Fri)
For urgent medical advice, contact NHS 111 or go to your nearest Emergency Department.