Entonox is a medical gas containing a mixture of 50% nitrous oxide and 50% oxygen.

Entonox can help patients be handled and moved with minimal pain. Within the Immediate Care in Rugby setting (once checking contraindications) it is an ideal analgesic to assist in the safe management of fractures or dislocations.

Entonox cylinders are identified by having a blue and white cap. If the gas cylinder has a Schrader valve attachment this makes the gas only usable with demand giving sets


Entonox is administered under medical supervision. The gas itself is self administered through a demand valve, using a mouth piece, bite-block or face mask. The demand valve opens when a small negative pressure has been generated by the patient’s individual effort. The Nitrous oxide is then exhaled via the expiratory effort port of the same valve.

The inspiratory resistance provided by the valve limits the use of the apparatus to children of approximately 5 years of age and over.

To administer entonox to smaller children requires an anaesthetic’ T’piece circuit in which gas flow is continuous. As a result this is not self administering and requires the presence of an anaesthetist.

Administering entonox by this method is safe because if enough entonox is inhaled to induce a reduced level of consciouness the patient will be unable to hold the valve and drop the valve thus exhaling residual gas. The mixture of 50 % oxygen ensures the patient has sufficient oxygen in their system.

To administer Entonox the patient must be fully conscious and capable of understanding what is being offered. The patients airway must be clear and the patient be able enough to take a deep breath .It is important to warn the patient they may become drowsy taking only four or five breaths to produce an analgesic effect.

Nitrous oxide is more soluble than oxygen and nitrogen will diffuse in to air spaces within the body. This makes it dangerous to use in patients with suspected pneumothorax or who have recently been scuba diving. Similarly caution is required with patients with bowel obstruction or head injuries, since nitrous oxide will increase the pressure effect in these conditions.

Contraindications for the use of Entonox

  • Head Injuries and ↓GCS: Increasing the possibility of drowsiness or loss of consciousness and possibly expanding an aerocele.
  • Chest Injuries: Entonox is 34 times more soluble than nitrogen. Consequently it diffuses in to cavities more rapidly than nitrogen diffuses out .This will cause an increase in the volume of space and an increase in pressure effect of a gas filled cavity. Thus increasing the risk of a pneumothorax and the conversion of a closed pneumothorax to a tension pneumothorax.
  • Air Embolism: increased expansion
  • Maxillofacial Injuries: increased risk of surgical emphysema and blow out fracture.
  • Decompression Sickness: due to increased solubility of nitrogen bubbles in bloodstream
  • Intoxication: May increase the effects of alcohol or drugs.

Travelling abroad

When using entonox abroad a variety of valve attachments may be required different to those used in England.

Similarly Entonox outside the Pre-hospital care environment is illegal in certain countries, hence if travelling abroad with a sports team it is important to check the medical regulations for that country regarding possession of entonox.

An alternative to Entonox is Penthrox (methoxyflurane) for the emergency relief of moderate to severe pain in conscious adults with trauma associated pain.