Nitrous Oxide pre-mixed 50/50 with Oxygen (Entonox® or Nitronox®)

Section 1.1

Midwives may independently prescribe, order and administer this drug.

Premixed 50/50 concentration of nitrous oxide and oxygen.

Nitrous oxide increases cerebral blood flow and intracranial pressure and decreases hepatic and renal blood flow; analgesic action is similar to morphine.

Indications and Clinical Use:

For relief of moderate pain in normal labour. Commonly used in the transition period prior to full dilation. Its analgesic effect is strong and characterized by rapid onset and offset.


Nitrous oxide should not be used in patients with bowel obstruction, pneumothorax, middle ear or sinus disease.

Hypersensitivity to nitrous oxide or any component of the formulation.
Nitrous oxide should not be used without oxygen.

Warnings and Precautions:

Must be self-administered in labour during contractions.
Oxygen use: Oxygen should be briefly administered during emergence from prolonged anesthesia with nitrous oxide to prevent diffusion hypoxia.


Human and Animal Data Suggest Risk


Category L3 – No Data – Probably Compatible

Adverse Reactions:

Hypotension, headache, dizziness, confusion; possibly nausea and vomiting.

Personnel exposed to unscavenged nitrous oxide are at increased risk of renal and hepatic diseases and peripheral neuropathy as seen in vitamin B12 deficiency. Dental personnel exposed to unscavenged nitrous oxide for more than 5 hours/week are significantly less fertile than those who are not exposed, or who are exposed to lower levels of scavenged or unscavenged nitrous oxide.

Dosage and Administration:

Face mask or mouth piece.
Self-administration should be at the beginning of a contraction.

Onset of Action:

Peak action: 30 seconds following administration.


3 minutes approximately