Nitrous oxide
Dinitrogen monoxide (N2O)
Also known as: Entonox · Laughing gas · N2O · Dinitrogen monoxide · Nitrogen oxide
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunct for induction and maintenance of anaesthesia | Inspired concentrations of 50-70% | Inhalational | Continuous during anaesthesia | Duration of procedure | 🌍 EU |
- Adjunct for induction and maintenance of anaesthesia: Must be administered with a minimum of 30% oxygen.
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Adjunct for induction and maintenance of anaesthesia | Inspired concentrations of 50-70% | Inhalational | Continuous during anaesthesia | Duration of procedure | 🌍 EU |
- Adjunct for induction and maintenance of anaesthesia: Must be administered with a minimum of 30% oxygen.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Nitrous oxide (N2O) is an inhalational gas used in combination with oxygen to carry volatile anaesthetic agents (such as isoflurane) for the induction and maintenance of general anaesthesia. It reduces the required concentration of the primary inhalant agent and hastens the attainment of a stable anaesthetic plane via the second gas effect and concentration effect.
Clinical Warning: N2O will cause a rapid expansion of any gas-filled space, increasing volume or pressure. It is strictly contraindicated in patients with pneumothorax or gastric dilatation-volvulus (GDV).
Clinical Warning: Oxygen must be supplemented for 5-10 minutes after N2O is discontinued to prevent diffusion hypoxia.
Mechanism of action
Nitrous oxide causes generalized CNS depression.
When administered at the beginning of volatile agent anaesthesia, it increases the speed of uptake of the volatile agent from the alveoli → second gas effect and concentration effect → hastens attainment of a stable plane of anaesthesia. It has minimal direct respiratory and cardiovascular effects.
Safety & warnings
Contraindications
- Patients with air-filled spaces within the body (e.g., pneumothorax, gastric dilatation/GDV, intestinal obstruction)
- Marked respiratory compromise (due to hypoxia risks)
- Raised intracranial pressure (due to associated increases in cerebral blood flow)
Adverse effects
- Diffusion hypoxia (if oxygen is not supplemented upon discontinuation)
- Megaloblastic changes in bone marrow (with exposure lasting a few hours)
- Agranulocytosis (with prolonged exposure of a few days)
Precautions
Human Safety & Scavenging: Prolonged exposure can have serious adverse effects on human health. Active scavenging is essential.
Important: N2O is not absorbed by activated charcoal in passive scavenging systems.
Hypoxia Risk: A minimum oxygen concentration of 30% is required during anaesthesia. The inspired concentration of oxygen may fall to critically low levels when N2O is used in rebreathing circuits during low flow rates. Do not use in such systems unless the inspired oxygen concentration can be measured on a breath-by-breath basis.
Diffusion Hypoxia: Always supplement with 100% oxygen for 5-10 minutes after turning off N2O.
Monitoring
- Inspired oxygen concentration (FiO2) - especially in rebreathing circuits
- Pulse oximetry (SpO2)
- Capnography (ETCO2)
- Respiratory rate and effort
- Cardiovascular parameters (HR, BP)
Pharmacokinetics
Half-life
Absorption
Rapidly absorbed across the alveolar membrane into the blood.
Distribution
Distributes rapidly to highly perfused tissues including the brain.
Metabolism
Virtually no biotransformation; the cobalt ion in vitamin B12 is oxidized by N2O, inhibiting methionine synthase.
Elimination
Rapidly excreted unchanged via the lungs upon discontinuation.
Overdose
Overdosage primarily presents as hypoxia if the inspired oxygen concentration falls below 30%. Treatment consists of immediately discontinuing nitrous oxide and providing 100% oxygen via positive pressure ventilation if necessary.
Available products
Formulations
- 100% nitrous oxide (N2O) gas
- Nitrous oxide plus oxygen (Entonox)
Veterinary
- Nitrous oxide medical gas cylinders
Human-labeled
- Entonox (50% N2O / 50% O2) cylinders
- Nitrous oxide medical gas cylinders
Regulatory status
Regulated as a medical gas.
POM (Prescription Only Medicine)
POM-V. Strict regulations regarding occupational exposure and scavenging apply.
No regulatory data for: 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store in pressurized medical gas cylinders in a cool, dry, well-ventilated area away from combustible materials. Cylinders must be secured to prevent falling.
Client information
Nitrous oxide (often called 'laughing gas') is a safe and commonly used gas that veterinarians use alongside oxygen and other anesthetics to keep your pet asleep and pain-free during surgery. It helps the main anesthetic work faster and allows the veterinarian to use lower doses of other drugs. It leaves the body very quickly once the procedure is over, allowing for a smooth recovery.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.