Inhalational Analgesic — Procedural Pain Pregnancy: Avoid in first trimester — B12 metabolism effects; widely used in labour analgesia after 16 weeks

Entonox (Nitrous Oxide 50% / Oxygen 50%)

Brand names: Entonox, Equanox

Adult dose

Dose: Inhale on demand via patient-controlled mouthpiece or mask; onset 30–60 seconds

Route: Inhalational (self-administered)

Frequency: As needed during procedure

Max: Not for use >6 hours continuously; not more than weekly

Patient-controlled inhalational analgesia for burns dressing changes — fast onset, rapid offset (2–5 min). Self-limiting — patient loses consciousness before dangerous levels reached (drops mask). Do NOT use in enclosed spaces. Requires well-ventilated area or scavenging system.

Paediatric dose

Route: Inhalational

Frequency: Self-administered

Max: As adult

Children ≥5 years: can be taught self-administration via mouthpiece. Under 5 years: too young to self-administer reliably — use alternative procedural sedation.

Dose adjustments

Renal

No adjustment required.

Hepatic

No adjustment required.

Clinical pearls

Self-limiting safety mechanism: patient drops mask if they become too sedated — inherent safety feature
Do NOT use >6 hours continuously — irreversible inactivation of methionine synthase causes functional B12 deficiency and bone marrow suppression (myeloneuropathy with chronic exposure)
Check B12 levels and supplement in burns patients receiving frequent Entonox — particularly important in malnourished or vegetarian patients

Contraindications

Pneumothorax
Bowel obstruction
Severe bullous emphysema
Middle ear or sinus surgery within recent weeks (air space expansion)
Vitamin B12 deficiency (N2O irreversibly oxidises B12 — contraindicated)
Recent intraocular gas injection (SF6, C3F8)
Decompression illness

Side effects

Dizziness and lightheadedness
Nausea and vomiting (~20%)
Euphoria
Tingling (perioral, limbs)
Bone marrow suppression (repeated frequent use — B12/folate metabolism)
Subacute combined degeneration of spinal cord (chronic occupational exposure)

Interactions

Methotrexate (additive folate antagonism — avoid)
B12 supplements required for frequent use

Monitoring

B12 and folate levels in patients receiving repeated use
Adequate scavenging or ventilation (staff occupational health)
Pain and anxiety scores pre/post procedure

Reference: BNFc; BNF 90; AAGBI/Faculty of Pain Medicine Entonox Guidelines; BBA Burns Dressing Change Analgesia Protocol. Verify against your local formulary and the latest BNF before prescribing.

Curated clinical cross-links plus same-class fallbacks.

Calculators