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Penthrox 99.9% 3 ml inhalation Vapour liquid – patient leaflet, side effects, dosage

Text from the MHRA, as of · About this document

Medicine status: Prescription only (POM) Details

Medicine strength:
99.9%
Contains active substance:

Patient leaflet - Penthrox 99.9% 3 ml inhalation Vapour liquid

1. What Penthrox® is and what it is used for

Penthrox® contains the active substance methoxyflurane. It is a medicine which is used to reduce pain. It is inhaled through the Penthrox® Inhaler. The intended purpose of the Penthrox® Inhaler is to administer the analgesic drug Penthrox® (methoxyflurane). Penthrox® is poured into the device and as you breathe in through the device you inhale the Penthrox® vapour. The attached Activated Carbon (AC) Chamber adsorbs the Penthrox® vapour you are exhaling through the Penthrox® Inhaler. Penthrox® administered through the Penthrox® Inhaler, is intended to reduce the severity of pain, rather than stop it completely.

2. What you need to know before you use Penthrox®

Do not use this medicine if you:

  • are allergic to methoxyflurane, any inhalation anaesthetics or any of the other ingredients of this medicine (listed in section 6).
  • have a history or family history of malignant hyperthermia. Malignant hyperthermia is a condition where symptoms such as a very high fever, fast, irregular heartbeat, muscle spasms and breathing problems have occurred after you, or a related family member, have been given an anaesthetic
  • or your family have a history of severe side effects to inhaled anaesthetics
  • have previously had liver damage after using methoxyflurane or any inhalation anaesthetics
  • have significant kidney impairment
  • have a change in the level of consciousness due to any cause including head injury, drugs, or alcohol
  • have significant heart or circulatory problems
  • have shallow breathing or difficulty in breathing This medicine must not be used as an anaesthetic. If you are not sure whether you should be given this medicine, talk to your healthcare professional. Warnings and precautions

Talk to your healthcare professional before using this medicine if you:

  • have liver or kidney problems
  • have a medical condition which may cause kidney problems
  • are elderly

Respiratory depression, with symptoms such as too slow and shallow breathing or other difficulties in breathing, has been reported in association with treatment with Penthrox® (section 4). Tell your healthcare professional immediately if you experience any breathing problems. Children

Do not give this medicine to children.

Other medicines and Penthrox®

Tell your healthcare professional if you are taking or have recently taken any other medicines. In particular, tell your healthcare professional if you are taking any of the following:

  • Isoniazid to treat tuberculosis
  • Phenobarbital or carbamazepine to treat epilepsy
  • Rifampicin or other antibiotics to treat infection
  • Medicines, or illegal drugs, that have a dampening effect on the nervous system such as narcotics, pain killers, sedatives, sleeping pills, general anaesthetics, phenothiazines, tranquilisers, muscle relaxants and sedating antihistamines.
  • Antibiotics and other medicines that may harm the kidney such as tetracycline, gentamicin, colistin, polymyxin B, amphotericin B and contrast agents.
  • Efavirenz or nevirapine to treat HIV

Ask your healthcare professional if you are unsure. If you need hospital treatment requiring general anaesthesia, tell the doctors treating you that you have

used this medicine.

Penthrox® with food, drink and alcohol

Do not drink alcohol whilst taking this medicine as it may increase its effect.

You can eat and drink as normal whilst taking this medicine, unless advised by your healthcare professional. Pregnancy, breast-feeding and fertility

Tell your healthcare professional before taking this medicine if you are pregnant, intending to become pregnant, are breast-feeding or intending to breast-feed. Your healthcare professional will discuss the possible risks and benefits of being given this medicine. Driving and using machines

This medicine can affect your ability to drive or use machines safely. Make sure these abilities are not affected before you drive or operate machinery. Penthrox® may cause drowsiness or dizziness in some people. Penthrox® contains Butylated hydroxytoluene (E321)

This medicine contains a stabiliser ingredient called butylated hydroxytoluene (E321). Butylated hydroxytoluene may cause local skin reactions (e.g.contact dermatitis), or irritation to the eyes and mucous membranes.

3. How to use Penthrox®

Always take this medicine exactly as your healthcare professional has told you. Check with your healthcare professional if you are not sure.

Adults

One or two bottles containing 3 ml of Penthrox® can be used per administration. The maximum dose is two bottles containing 3 ml per administration. Do not inhale more than the maximum dose. The Penthrox® Inhaler is designed for use with Penthrox® (methoxyflurane) only. How to use Penthrox®

  1. 1.

    Your healthcare professional will

    prepare the Penthrox® inhaler and place the wrist loop over your wrist.

  2. 2.

    Breathe in through the mouthpiece of the inhaler to obtain pain relief. Your healthcare professional will show

    you how if you are unsure. Accustom yourself to the fruity smell of the

    medicine by inhaling gently for the first few breaths. Breathe out through the inhaler. After the first few breaths breathe normally through the inhaler.

  3. 3.

    If you need stronger pain relief, cover the dilutor hole on the transparent

    chamber with charcoal with your finger during use. Your healthcare professional will show you where the hole is.

  4. 4.

    You do not need to breathe in and out of the inhaler all of the time. Your

    healthcare professional will encourage you to take breaks from the inhaler as this will make the pain relief last longer.

  5. 5. Continue using your inhaler until your healthcare professional tells you to stop, or when you have inhaled the maximum recommended dose.

Do not give this medicine to anyone else, even if they have the same condition as you.

If you use more Penthrox® than you should

The healthcare professional giving the medicine will be experienced in its use, so it is extremely unlikely that you will be given too much. You should not use more than 2 bottles at one time. If the maximum dose is exceeded, Penthrox® may cause damage to your kidneys. Tell your healthcare professional immediately if you think you may have taken too much of this medicine.

If you have any further questions on the use of this medicine, ask your healthcare professional.

4. Possible Side Effects

Like all medicines, this medicine can cause side effects, although not everybody gets them.

Serious side effects

Tell your healthcare professional immediately if you experience any of the following:

  • Serious allergic reaction, symptoms include difficulty breathing and/or swelling of the face
  • Liver problems, such as loss of appetite, nausea, vomiting, jaundice (yellowing of the skin and or eyes), dark coloured urine, pale coloured stools, pain/ ache or sensitivity to touch in your right stomach area (below your ribs)
  • Kidney problems such as reduced or excessive urination or swelling of feet or lower legs.
  • Too slow and shallow breathing or other difficulties in breathing (symptoms of respiratory depression) (frequency cannot be estimated from available data) The above side effects can be life threatening so tell your healthcare professional immediately. PACKAGE LEAFLET: INFORMATION FOR THE USER

Penthrox® 99.9% 3 ml inhalation vapour, liquid

methoxyflurane

AW-0182_LFT260-UK-V20-MDR+PVAR_Draft3 320mmX150mm

Other side effects

Very common side effects: may affect more than 1 in 10 people

  • Dizziness

Common side effects: may affect up to 1 in 10 people

  • Drowsiness
  • Headache or nausea
  • Feeling of extreme happiness
  • Feeling drunk
  • Taste disturbance
  • Coughing

Uncommon side effects: may affect up to 1 in 100 people

  • Anxiety or depression
  • Disturbance in attention
  • Inappropriate emotions or actions
  • Repetition of words or difficulty in speaking
  • Memory loss
  • Tingling skin (pins and needles) in the hands or feet
  • Numbness
  • Vision impairment
  • Flushing of the skin
  • Low or high blood pressure
  • Dry mouth
  • Mouth discomfort or itch
  • Increased salivation
  • Increased appetite
  • Vomiting
  • Sweating
  • Tiredness
  • Feeling abnormal
  • Chills
  • Feeling relaxed

Not known: frequency cannot be estimated from the available data

  • Hypersensitivity
  • Mood swings
  • Restlessness or agitation
  • Feeling of being disconnected from reality
  • Disorientation
  • Altered state of consciousness
  • Choking
  • Shortness of breath
  • Uncontrolled eye movements

    Reporting of side effects:

    If you get any side effects, talk to your healthcare professional. This includes any possible side effects not listed in this leaflet. You can also report side effects directly via the Yellow Card Scheme at

    www.mhra.gov.uk/yellowcard. By reporting side effects, you can help provide more information on the safety of this medicine.

5. How to store Penthrox®

Keep this medicine out of the sight and reach of children. Do not use this medicine after the expiry date which is stated on the carton after EXP. The expiry date refers to the last day of that month.

This medicinal product does not require any special storage conditions. Your healthcare professional will keep Penthrox® combination pack in a locked cabinet, rather than an open shelf.

Your healthcare professional will dispose of any leftover Penthrox® liquid and the Penthrox® Inhaler in the appropriate way.

6. Contents of the pack and other information

What Penthrox® contains

The active substance is Methoxyflurane. Each sealed bottle contains 3 ml methoxyflurane 99.9%. The other ingredient is Butylated hydroxytoluene (E321). What Penthrox® looks like and contents of the pack

Penthrox® is a clear, almost colourless volatile liquid, with a characteristic fruity odour that becomes a vapour or gas when used with the Penthrox® Inhaler.

Penthrox® is supplied in the following presentations:

  1. a) One bottle with a tear off tamper-evident seal (packs of 10), or
  2. b) Combination pack with one bottle of 3 ml Penthrox®, one Penthrox® inhaler and one Activated Carbon (AC) chamber (packs of 1 or 10).

Not all pack sizes may be marketed.

Marketing Authorisation Holder and Manufacturer Marketing Authorisation Holder

Medical Developments UK Ltd.

Causeway House

1 Dane Street,

Bishop’s Stortford

Herts CM23 3BT

United Kingdom

Manufacturer

MIAS Pharma Limited

Suite 1, First Floor, Stafford House,

Strand Road Portmarnock,

Co. Dublin D13 WC83 Ireland

Distributor

Galen Limited

Seagoe Industrial Estate

Craigavon BT63 5UA

United Kingdom

This leaflet was last revised in

The physician, doctor, nurse, paramedic, or person trained in administering, Penthrox® must provide and explain the Package Leaflet to the patient.

  1. 1.

    Ensure the Activated Carbon (AC)

    Chamber is inserted into the dilutor hole on the top of the Penthrox® inhaler.

  2. 2.

    Remove the cap of the bottle by

    hand. Alternatively, use the base

    of the Penthrox® inhaler to loosen

    the cap with a ½ turn. Separate the

    inhaler from the bottle and remove the cap by hand.

  3. 3.

    Tilt the Penthrox® inhaler to a 45°

    angle and pour the total contents of one Penthrox® bottle into the base of the inhaler whilst rotating.

  4. 4. Place wrist loop over patient’s wrist. Patient inhales through the mouthpiece of Penthrox® inhaler to obtain analgesia. First few breaths should be gentle and then breathe normally through inhaler.
  5. 5.

    Patient exhales into the Penthrox®

    inhaler. The exhaled vapour passes

    through the AC Chamber to adsorb

    any exhaled methoxyflurane.

    1. 6.

      If stronger analgesia is required, patient can cover dilutor hole on the AC

      chamber with finger during use.

    2. 7. If further pain relief is required, after the first bottle has been used use a second bottle if available. Alternatively use a second bottle from a new combination pack. Use in the same way as the first bottle in step 2 and 3. No need to remove the AC Chamber. Put used bottle into the plastic bag provided.
    3. 8.

      Patient should be instructed to inhale

      intermittently to achieve adequate

      analgesia. Continuous inhalation will

      reduce duration of use. Minimum dose to achieve analgesia should be administered.

    4. 9.

      Replace cap onto Penthrox® bottle. Place used Penthrox® inhaler and used bottle

      in sealed plastic bag and dispose of

      responsibly.

Information for Healthcare Professionals

The following information is intended for healthcare professionals only: Instructions on the preparation of the Penthrox® inhaler and correct administration are provided in the Figures below:

Before use, check for foreign objects, broken or deteriorated parts. Do not use this device if any are identified.

LFT260-UK-V20

About this document

Source: MHRA - Medicines and Healthcare products Regulatory Agency (products.mhra.gov.uk) · Marketing authorisation: PL 42467/0001

MHRA data as of 13 October 2026

Found a mistake? Tell us on the contact page.

The information on this site does not replace the advice of a doctor or pharmacist.

Facts about Penthrox 99.9% 3 ml inhalation Vapour liquid

Prescription needed? Yes, prescription-only medicine (POM)
Active substance Methoxyflurane
Strength 99.9%
Pharmaceutical form Clear, almost colourless, volatile liquid, with a characteristic fruity odour.
Excipients Butylated hydroxytoluene E321 (stabiliser).
Is it available? Listed as available (NHS Dictionary of Medicines and Devices – not a record of current supplies)
Shelf life 36 months.
Storage This medicinal product does not require any special storage conditions. For storage, PENTHROX combination pack should be kept in a locked cabinet, and should not be left on an open shelf.
Drug group (ATC) Analgesics, other analgesics and antipyretics
  1. N Nervous system drugs
    1. N02 Analgesics
      1. N02B Other analgesics and antipyretics
        1. N02BG Other analgesics and antipyretics
          1. N02BG09 methoxyflurane
Marketing authorisation PL 42467/0001 (first authorised 11 November 2025)
Marketing authorisation holder Medical Developments UK Limited

Sources: the MHRA product information (products.mhra.gov.uk) and the NHS Dictionary of Medicines and Devices (dm+d), release 10.2.0 of 19 October 2026, NHS Business Services Authority. Contains public sector information licensed under the Open Government Licence v3.0.