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Inhalation anaesthetic / analgesic

Nitrous oxide

Brand names: Entonox (50% N2O / 50% O2)

Nitrous oxide is an inhaled gas used as a weak anaesthetic adjunct and as an analgesic, including in a fixed nitrous oxide/oxygen mixture for procedural pain and in labour.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

Clinical monograph

How it works

Its analgesic and anaesthetic effects are mediated largely through antagonism of NMDA glutamate receptors, with additional opioid and other neuromodulatory actions.

Prescribing in practice

  • It diffuses into air-filled spaces and must be avoided where expansion is dangerous, such as pneumothorax, bowel obstruction, recent intraocular gas, or air embolism.
  • Prolonged or repeated exposure inactivates vitamin B12, impairing folate metabolism and causing megaloblastic changes and, rarely, neurological harm.
  • When discontinued, give supplemental oxygen to avoid diffusion hypoxia.

Monitoring

Monitor oxygenation and conscious level during use, and consider vitamin B12 status with prolonged or frequent exposure.

Counselling the patient

  • You may feel light-headed, drowsy, or nauseated while breathing the gas; these effects pass quickly afterwards.
  • Tell the team if you have had recent eye surgery involving gas, or any condition with trapped air.

Evidence & guidelines

Use reflects established anaesthetic and procedural analgesia practice; consult current prescribing references and the SPC.

Reference: AAGBI; MHRA; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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